Microsoft Security Essentials ^[1]

Limited to personal or home-office use and small business up to 10 PCs only. Requires genuine Windows.

Features

• No registration, no ads
• Simple, unobtrusive UI
• Updates via Microsoft Update (note: virus definitions are updated more often)

Now extremely widely used and well tested!

avast! Free Antivirus ^[1]

Free for home use only.

Features

• Very Fast ^[2]
• Very small, fast updates (highly recommended for those on dial up)
• Lightweight ^[3]
• Updates released really often (if it is needed, immediately)
• Boot-time scans
• Voice notifications
• No ads

Drawbacks

• Frequent audio/popup notifications may be annoying. (Can be disabled)
• Email registration required. (They haven't to date abused this, though.)
• Bundled software (Google Chrome; optional during install.)

Other

• A lot of custom voices ^[4].

Avira AntiVir Personal ^[1]

Limited to personal use.

Features

• No registration required
• Unobtrusive and reliable
• Good customer support
• Regular updates

Drawbacks

• Displays/nags ads after each update of the virus definitions

Extras

• Kill the ad nagging by adding a software restriction for avnotify.exe

AVG ^[1]

With regards to the second part of the question:

Limitations:

• Support. You're unlikely to get support from the provider, and if you do, it's likely to be of lower quality (e.g. slower) than for a paid product. For most users, I doubt support for anti-virus software is that important, but some people like to have a number to call for everything.
• Usage restrictions. Most free solutions are limited to personal use only. For example, AVG's free edition is limited to "private, non-commercial" use only, which means you can't (legally) use it on your work computer, even if you're a small business.
• Functionality. Often free versions have more limited functionality. Using AVG as an example again, their paid versions have IM, identify theft, spam, and rootkit protection, and a firewall.
• Ads. Often a free product will incorporate some form of advertising (even if it's just for a paid version of the same product). This can range from a subtle image in the UI (which you'll rarely use), to something annoying.

As for dangers, I presume you mean "are these products less likely to catch viruses?". Perhaps. You can look at evaluations (e.g. the Virus Bulletin ones ^[2]), although some people argue that those aren't particularly meaningful. In my experience, if you keep whatever product you use up-to-date (and the definitions up-to-date as well), and you have a firewall, and you're behind some sort of NAT (e.g. most home connections), and you keep your OS up-to-date, and you don't behave stupidly, then you have little risk.

FWIW, despite these limitations, I always recommend free anti-virus to friends/family (typically AVG). For the commercial situation, there are many advantages in the commercial products, but there are plenty of good free choices for personal, home, use.

ClamWin ^[1]

An open source product.

Features

• Database updates pretty fast with new virus definitions
• Outlook plug-in allows you to scan incoming and outgoing emails

• Can be set to work with Firefox to scan downloads with a Download Statusbar add-on. Add-on has to call ClamWin.exe on the command line with the following:

  "--mode=scanner --remove --close --path=%1"

  which will remove any flagged and closes after each download

Drawbacks

• Current version does not do real-time scanning. It is planned for its next major release version ^[2]

Antivirus software? Never! I just keep images of my system in a fresh state and backups of my data. Reload the system every 3-6 months to keep it fresh and snappy.

But if you must use AV software, I recommend Kaspersky or AVG.

Comodo ^[1]

Current free version edition combines the antivirus and firewall. You can opt out of either component if you're already using or want to use another vendor.

Features

• Can be set up to constantly nag you about programs trying to access the Internet without explicit permission
• Also has one of the best free firewalls as option to install with
• Set up zones outlining what software is allowed to access what of the computer, registry and memory, etc.

Drawbacks

• Occasionally aggressive malware detection flagging false-positives
• AV isn't Windows certified - Vista's security centre won't recognise it as one of the options if you don't use its tools
• Classified as "adware" by Softpedia ^[2] for the hints at install at trying to plant a SafeSearch browser toolbar (superfluous to program running)
• Every now and then the virus database update might hobble your entire system ^[3].

Supported systems

• Windows XP (SP2) - 32 and 64 bit versions
• Windows Vista/7 - 32 and 64 bit versions

Malwarebytes' Anti-Malware ^[1]

An anti-malware application that can thoroughly remove even the most advanced malware.

The free version doesn't have real time protection but it's a good alternative to make complementary on-demand scans.

VirusTotal.com

VirusTotal is a service that analyzes suspicious files and facilitates the quick detection of viruses, worms, trojans, and all kinds of malware detected by antivirus engines.

VirusTotal Uploader ^[2] enables you to directly send files from your system using the context menu.

Specs:

• Free, independent service
• Use of multiple antivirus engines
• Real-time automatic updates of virus signatures
• Detailed results from each antivirus engine
• Real time global statistics

Pros:

• Web Based
• Scans files using free/open/commercial scanning engines
• compares results
• Lists a bunch of other interesting properties about the files

Cons

• Web based
• Single file upload
• Potential size restrictions

Panda Cloud Antivirus ^[1]

The first antivirus based on this innovative protection model which is based on two fundamental principles:

1. Automatic malware detection and remediation from the cloud in real-time.
2. The use of an ultra-lightweight thin-client agent that off-loads the hard work to the server

Dr. Web CureIt! ^[1]

Not a true resident antivirus suite but it's good tool to make an on-demand scan.

There's also a Live CD ^[2] version that could be useful

Features

• No install required, comes as a self-contained EXE

Drawbacks

• Nagware

HouseCall ^[1]

From Trend Micro

Features

• Web based

Drawbacks

• Web based

Kaspersky Virus Removal Tool ^[1]

Not a true resident antivirus suite but it's one of the best tool to make an on-demand scan.

Kaspersky® Virus Removal Tool is an utility designed to remove all types of infections from your computer. Kaspersky Virus Removal Tool implies effective algorithms of detection used by Kaspersky Anti-Virus ^[2] and AVZ ^[3].

Kaspersky Virus Removal Tool is not capable for real-time protection of your computer. As soon as your computer is cleaned you are supposed to remove the tool and install a full version of antivirus software.

Highlights

• Easy interface.
• Can be installed to an infected machine (Safe Mode supported).
• Integral search and removal of malicious software: effective combination of signature detection and heuristic analyzer.
• System Analysis and interactive scripting language.

Basic functions

• Automatic and manual removal of viruses, Trojans and worms.
• Automatic and manual removal of Spyware and Adware modules.
• Automatic and manual removal of all types of rootkits.

• System memory is not available in x64 OSes Windows XP / Vista / 7 because of work specifications of the application's system drives.
• if User Account Control is enabled in Windows Vista settings, then it is impossible to rename a folder with application's files if Self-Defence is disabled.

I'm quite happy with Avira AntiVir ^[1] free for all of the home systems I have to administer. Lately, I'm personally running the new Microsoft Security Essentials ^[2]. It's VERY low load and updates several times a day.

I've always been intrigued with Threatfire ^[1]. I've installed it on a couple virtual machines but rarely encounter viruses in my daily life so I am not sure how effective a solution it is.

AntiVir ^[1] out of Germany. Updates are very regular and detection has been very good. Yes, the free version is limited to personal use. The only drawback is when it updates, a nagware screen shows with an OK button. Just click [Ok].

PrevX ^[1]

Although not a complete antivirus product (I use it in addition to Avira)

Features

• Scans a system very quickly
• produces good results even when running it on an infected system (thanks to the rootkit detector)
• can scan a system where another antivirus product is installed
• tends to detect some exotic malware that is not found by Avira/Avast

Drawbacks

• Will not find malware in every file, but only in places where the real malware will copy itself to. I.e. it is not sufficient for scanning your backups.
• Cannot disinfect the system (unless you can just delete the files manually)

The best free antivirus I ever used is Avira Antivir ( http://www.antivir.es/cms/ ), the virus found ratio is much better than AVG or Avast.

Secunia PSI ^[1]

It's not an antivirus, per se, but it's fantastic software for seeing what apps on your machine are vulnerable and thus preventing viruses from having an attack point.

What I particularly love about it is that there is a Simple and an Advanced interface. The simple interface makes it easy for non-tech users to just click to fix. In both interfaces, it's easy to click to go directly to the download of whatever software needs updating (which is very, very convenient).

Consider it!

I personally use ZenOK Free Antivirus its not just an AV it also antispyware, anti-spyware, ultra-light scanning, boot-up Protection a combined made a complete security suite protection.

A-squared Commandline Scanner ^[1] for me, like ClamAv/ClamWin no on-access protection. despite using a dual scan engine (Ikarus) for Viruses and Malware/Spyware it is probably the fastest virus scanner there is. extraordinary high detection rate ^[2].

free for personal use.

Spyware Terminator ^[1]

I use this on all of my computers and it works very well. It's free for personal and commercial use. It integrates with ClamAV, and is not very resource consuming either.

At work, we experimented with Malwarebytes, ClamWin, and Comodo, but ultimately the first 2 don't do real-time scanning and Comodo's AV was too rife with false positives, and isn't windows certified, so it won't turn off the security-center AV nag, though it can be disabled by GPO (in Comodo's defense, their free firewall continues to be great). As a result, we shelled out for a commercial anti-malware suite.

If I had to use only free products at work, I'd install Comodo and SpybotS&D/TeaTimer, scan regularly with ClamWin and/or Malwarebytes, use OpenDNS, and keep my fingers crossed.

Sometimes local viruses doesn't get detected by software like AVG or AntiVir, for those who lived in Indonesia PCMAV ^[1] seems to be a free but works solution.

I switched from Avast to Antivir, seems to be a bit more slim No registration needed so no key that expires, not quite sure if it catches more, as I don't come across many virii.

There is also Comodo BOclean ^[1], which is anti-malware, and seems to do it's business quietly.

I personally use AVG Free ^[1] combined with Spyware Doctor which is included in Google Pack ^[2]

As kristof said AVG free is a good choice and also i would highly recommend spy-bot search and destroy ^[1] (Don't be afraid of the terrible site design :) )

Over the course of a long computer support career I've used McAfee, Norton/Symantec and others briefly at work. I've moved to using only freeware solutions at home and have not had any problems. I've tested many - Avast, AVG, AntiVir, Clam and others. I typically use Acast as my freeware anti-virus of choice due to its reliability, effectiveness and ease of use. The only drawback is the need to register once a year (free). Overall for security on my home systems I've been using a combination of [Avast][1], [Threatfire][2], [Comodo][3] (firewall), [Dr. Web][4] in FireFox with occasional double-checks using [Spybot Search & Destroy][5] and Housecall online anti-virus. It's always a good idea to periodically double-check your solutions by rescanning with another solution. Over time some will catch things that others miss.

http://www.avast.com/eng/download-avast-home.html

Hmm. When I used XP, I liked F-Protect, but they seems to have amassed quite a lot of bloat lately. Most of the Windows people I know now use ESET's NOD32 ^[1].

http://www.consumersearch.com/antivirus-software

I will also say that Microsoft Security Essentials is the best free antivirus out there today. Being a computer technician I have seen this program in action from finding viruses on the drive to stopping them from being installed and I am very happy with it.

It also has a low footprint on your system so it doesnt slow down your computer.

PC Tools Spyware Doctor with Antivirus ^[1]

Or the more expensive PC Tools Internet Security 2011 ^[2] ( PC World's review ^[3])

Commercial/Shareware, $39.99. Here's PC World's 2010 review for Spyware Doctor only ^[4]. ( "Full" version ^[5].)

Features

• Easy to understand UI
• Comes with ThreatFire ^[6]
• In previous years, it had top real-time Malware detection rates, and was PC World's top editor's choice in Antivirus/Antimalware (in previous years, before Symantec bought it ;) Thankfully, they've decided to leave it alone, and not ruin "improve" it like their other products)
• Heuristics!
• Good Zero-Day Threat protection (or so they say)
• Ability to set Sensitivity level on detected threats; High/Medium-High sensitivity for geeks, and Low to Medium sensitivity for idiots ... err ... "normal people"

Drawbacks

• It may be the reason (5% sure) my system crashes once every two weeks on average, especially when I'm doing Power User/geeky stuff or when my system comes out of screensaver mode. (With "normal" everyday simple-user stuff, it's alright.)

VirtualBox ^[1]

While it's not a virus scanner per se, it it probably the best way to check if a suspicious file is indeed a virus.

I regularly come across files that are dubious at best, and the easiest way to absolutely tell if they're indeed malevolent is to simply run them. Running them in a VM prevents them from doing any damage, and you can simply roll-back the VM to a clean snapshot to remove the virus.

While it is a bit involved to get an OS up and running in a VM, now that I have it set up I find it tremendously useful for other things (It's a great way to keep stuff you use irregularly off your main OS).