The big three freeware antiviruses have traditionally being AntiVir, AVAST! and AVG. AntiVir in recent times has dominated some on-demand anti-virus tests, with results rivaling that of even top ranked antiviruses like Kaspersky Anti-Virus (KAV). AntiVir however lacks a email scanner unlike the other two, and the free version does not offer protection against spyware and adware. AVAST! provides additional features like Webshield (HTTP shield), IM/P2P shields on top of the standard resident shields. Experts differ on how important such features are (particularly the ability of http shield to handle exploits) given the existence of a classic file resident shield. AVG is probably the most popular of the three antiviruses, though it doesn't tend to do well in on-demand antivirus tests. AOL Active Virus Shield (discontinued) is based on KAV 6 (note: the latest KAV 6, provides various improvements, e.g PDM), and uses the well known and top rated antivirus engine. However many users have refused to use it on the account that it is from AOL. There is also some dispute over the permissive EULA. AOL has now replaced AOL Active Virus Shield with McAfee® VirusScan Plus (see below). This provides a more comprehensive range of protection covering email, IM as well as Buffer overflows and a system guard of various hijack points. There is also an option to install the whole suite including the firewall.
ClamAV is an open source project, it probably works best on email gateway servers rather than on host personal machines. The main project does not offer real time protection but there's a project to implement real time scanning or you could use Spyware Terminator with ClamAV.
Moon Secure Antivirus is yet another ambitious open source project but currently uses only ClamAV signatures for real time protection.
All the other freeware antiviruses are probably too new and are not mature enough (needs to catch up with databases) to be worth considering. Comodo AntiVirus is still immature, tests by AV-comparatives show it is still far below the levels of most conventional antivirus. It also incorporates executable whitelisting.
Malware Immunizer is an unorthodox (and probably ineffective) approach to preventing malware by creating harmless text files with the same names of files created by malware.
One problem with running multiple anti viruses is that conflicts can sometimes occur.Some antiviruses will not install if they detect the presence of another antivirus installed. Of course, you can use Online scanners but these have their own problems. Most of the antiviruses here (with the exception of BitDefender 10 Free Edition) are standalone executables that do not install any files or services (all resides in one directory) hence they can be used and removed without any problems. http://www.mwti.net/products/mwav/mwav.asp MWAV which uses the top notch KAV engine used to be popular, however it does not infect, and has annoying false positives with registry entries. Three worth considering are Dr.Web CureIt!, Norton Security Scan and BitDefender 10 Free Edition (though BitDefender is heavy and installs a lot of unnecessary services) because they are top notch antiviruses that can be used as backup scanners.
Mostly second tier or outdated products. F-PROT for Dos is respectable though development has stopped (signature updates are still available). Similarly Panda commandline scanner is meant for "research purposes" /"or those who wish to implement malware scanning in a not-for-profit project" only.
Generic tools to remove specific malware or groups of popular/common malware encountered. Use this, if you know what malware has infected the machine and/or you don't want or can't install the full antivirus packages. Multi-AV is particularly interesting since it is a program written that automates the handling of several of the programs listed here.
Note: This section is no longer updated, For updated list and more details about online scans including screenshots, file upload limits, ability to clean etc, see Online Scanners.
Online Scanners (full disk, some do not disinfect) Edit
VirusTotal Uploader, enables you to directly send files from your system using the context menu. NoSpyZone Security Center organises various security software into one interface for control.
CheckVir, ICSALabs, West Coast Labs checkmark certification are certifying organizations but not all antiviruses submit to such testing. Reputable (though this is often a matter of opinion) antivirus tests are generally on-demand tests, where the tester collects an archive of malware and the antiviruses are nade to scan it without running the malware. Examples include AV Comparatives, Avtest.org, Virus.gr . These generally include samples in the tens or hundreds of thousands and are done at monthly or longer periods. Historically the most famous and reputable test is VB100 awards, however this test differs from the others mentioned because of their relatively small sample size as they test mainly against malware that have being reported in the wild.
Malware Incident Reporting & Termination (MIRT) + OITC and Shadowserver virus stats differing from the tests already mentioned in that malware are tested against scanners (loaded on VirusTotal for the former) as and when they are reported as opposed to waiting and testing all of the them at the same time at fixed intervals (monthly or more). These tests appear to favour scanners with aggressive heuristics because most samples tests will be new or rare.AV Comparatives also provide retrospective tests where scanner signatures are held back for 3 months and tested against current malware to see how much is detected.
Malware-test lab actually executes and runs the malware first and then tests to see how
much of it is removed. The sample size is obviously much smaller. There are many other adhoc tests down on the net of course, but are of limited usefulness. See also this document.