What free antivirus solutions would you recommend?
As mentioned by Tony , most of the free solutions are limited to personal use so the question will mainly focus on solutions for personal use.
See if your antivirus of choice is already listed. Chances are it is.
If you spot an answer that mentions one you already use, vote that up if you think it's a good solution.
If you know of a feature or drawback not listed, or can include experiences in dealing with it, please edit the answer accordingly.
If you know of any that can also be used at work please point this out.
This covers all Windows platforms from XP, Vista and Windows 7. If you see an existing entry that needs an update or to add your testimonial, please do.
Limited to personal or home-office use and small business up to 10 PCs only. Requires genuine Windows.
Now extremely widely used and well tested! http://www.microsoft.com/security_essentials/
Free for home use only.
Limited to personal use.
With regards to the second part of the question:
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 ), 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. http://free.grisoft.com
An open source product.
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
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.
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.
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. http://www.malwarebytes.org/mbam.php
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  enables you to directly send files from your system using the context menu.
The first antivirus based on this innovative protection model which is based on two fundamental principles:
Not a true resident antivirus suite but it's good tool to make an on-demand scan.
There's also a Live CD  version that could be useful
From Trend Micro
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  and AVZ .
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.
I'm quite happy with Avira AntiVir  free for all of the home systems I have to administer. Lately, I'm personally running the new Microsoft Security Essentials . It's VERY low load and updates several times a day. http://free-av.com
I've always been intrigued with Threatfire . 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. http://www.threatfire.com/
AntiVir  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]. http://www.free-av.com/
Although not a complete antivirus product (I use it in addition to Avira)
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.
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! http://secunia.com/vulnerability_scanning/personal/
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  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 .
free for personal use. http://www.emsisoft.com/en/software/cmd/
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. http://www.spywareterminator.com/
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  seems to be a free but works solution. http://pcmav.biz/download
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 , which is anti-malware, and seems to do it's business quietly. http://www.comodo.com/boclean/boclean.html
As kristof said AVG free is a good choice and also i would highly recommend spy-bot search and destroy  (Don't be afraid of the terrible site design :) ) http://www.safer-networking.org
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], [Threatfire], [Comodo] (firewall), [Dr. Web] in FireFox with occasional double-checks using [Spybot Search & Destroy] 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.
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 .
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.
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). http://www.virtualbox.org/