Virus scanners operate by searching your harddisk(s) for files which might be infected by a virus. This method is never 100% secure, since only known viruses (or known virus patterns) are detected. If an infection has been detected in a certain file, the virus scanner might be able to clean the file, but very often this is not the case. In that case, you can try another scanner to clean the file, you can try to replace the file or - as a last resort - delete it. However, if many important operating system files are affected, it might be the best solution to reformat the harddisk and install the operating system from scratch. There are even viruses can not be cleaned by mere hard-disk reformatting. E.g. so-called boot-viruses infect the boot-sector of disks, and will very often not be removed by simply reformatting the harddisk. In this case you will need to repair the boot-sector, a good utility for this can be found in Norton Utilities (not free).
Antidote Super Lite version	Freeware lite version of the commercial Antidote program. It utilizes the same virus database as the commercial version.
Avast!	Anti-virus program for Windows. The home edition is freeware for noncommercial users.
AVG Free edition	Free edition of the AVG anti-virus program for Windows and Linux. Tested and recommended by Freebyte.com.
Avira Antivir	Free anti-virus software for Windows, Linux, Free BSD and Solaris. Detects and removes more than 50,000 viruses. Free support.
BitDefender	Freeware virus scanner for Linux.
Clam AV	Freeware, GPL, Linux. Clam AntiVirus is a GPL anti-virus toolkit for UNIX. The main purpose of this software is the integration with mail servers (attachment scanning). The package provides a flexible and scalable multi-threaded daemon, a command line scanner, and a tool for automatic updating via Internet.
ClamWin	Freeware, open-source anti-virus program for Windows.
Comodo Anti-Virus	Free anti-virus program for Windows.
Cyberhawk	Protects against viruses, worms, trojans and other spyware, designed to be a complement to your current anti-virus software. Cyberhawk does not need to know anything about a virus before it can protect you against it. This is because Cyberhawk looks at the behaviour of an infected program (instead of the contents of the infected file). The basic version is free. Note from the editors: after testing Cyberhawk we found that it generates a lot of false alarms on legitimate software. Nevertheless we think that this program can be useful provided you know how to interpret these alarms.
FProt	Free anti-virus software for Linux, FreeBSD and DOS (personal use). Evaluation version for Windows.
HandyBits	Free for personal use. Virus 'scanner integrator'. After performing an auto-search for installed virus scanners, it will scan your files using all found installed virus scanners. This can be useful, since some antiviral programs are good for one type of viruses, other programs for other viruses.
PC Tools AntiVirus	Free anti-virus program + firewall for Windows.