*AVG Free
AVG Anti-Virus Free Edition download and installation is only available for single computer use for home and non commercial use. It’s compatible with Windows 7, Windows Vista and Windows XP

*avast! Home Edition
avast! Antivirus Home Edition is FREE OF CHARGE for non-commercial & home use only.

*Avira AntiVir
Avira AntiVir FREE scans your computer for malicious programs such as viruses, Trojans, backdoor programs, hoaxes, worms, dialers etc. Available for Windows and Linux.

Basic virus protection for free. Available for Windows Vista and Windows XP

*Comodo Internet Security
Comodo Internet Security includes AntiVirus + Comodo Firewall. It has all the functionality of a paid AV without the price. It eliminates all known Viruses, Worms and Trojans from desktops and networks with no license fees. Available for Windows Vista and Windows XP.

Free anti-virus program for Linux, FreeBSD and Solaris (personal use only).

Free for personal use. With VirusScan Integrator you can scan the same files with several antiviral programs. This can be useful, since some antiviral programs are good for one type of viruses, other programs for other viruses.

*PC Tools Antivirus
With PC Tools AntiVirus Free Edition you are protected against the most nefarious cyber-threats attempting to gain access to your PC and personal information. Available for Windows Vista 32-bit, XP and 2000.

The basic version is free for personal home use. Offers real-time protection against both known and unknown threats. Available for Windows Vista/XP/2003/2008.

ClamWin Free Antivirus does not include an on-access real-time scanner. You need to manually scan a file in order to detect a virus or spyware. Available for Microsoft Windows 98/Me/2000/XP/2003 and Vista.

and some free online antivirus scanners

Online Trojan scanner. This program runs in your browser, no explicit installation is required.
Requirements: Windows 2003 Server, XP, 2000, ME, NT 4 or 98 and with only Internet Explorer 5.0 or later with ActiveX enabled.

Free online virus scanner and a free scanner to check a single file. The online virus scanner will not remove the malware from your machine if it finds it – Kaspersky software is required to do this.
Requirements: * Microsoft Windows 2000 Professional SP4, 32-bit / 64-bit Microsoft Windows XP SP2 or 32-bit / 64-bit Microsoft Windows Vista. Microsoft Internet Explorer 6 or 7, Opera 9 or Firefox 2;

*McAfee FreeScan
FreeScan searches for viruses, including the latest known “in the wild” viruses, and displays a detailed list of any infected files. Should viruses be found, FreeScan provides links to more information about the viruses and what you can do to clean your system.
Requirements: Microsoft Internet Explorer 5.0 or later

*Panda Active Scan
Panda ActiveScan 2.0 detects all types of viruses, Trojans, worms, spyware, dialers, hacking tools, jokes, and other active or inactive (latent) security risks on your PC. It also detects vulnerabilities of software installed on your PC which could compromise your computer’s security.
Requirements: Windows 2000/XP/Vista & Internet Explorer 6 or later. Mozilla Firefox 1.5 or later. Have Javascript execution enabled. Cookies enabled.

*Symantec Security Check
Symantec Security Check tests your computer’s exposure to a wide range of online threats, including a virus scan.
Requirements: Microsoft Internet Explorer 5.0 or higher with ActiveX and Scripting enabled.

*TrendMicro HouseCall
Trend Micro HouseCall is an online application that scans your PC for a wide range of Internet security threats, detects and cleans your computer of possible infection by viruses, worms, Trojans and spyware or other malware.
Requirements: Microsoft Windows 98SE/NT4.0,SP6a/2000,SP2/XP,SP1/2003 and Windows MCE 2005. Linux Distributions that supports libc6.Solaris 2.6 and above
Software: Microsoft Internet Explorer (IE) 6.0 or later.Mozilla Firefox 1.0.5, 1.0.6, 1.0.7, 1.5. Mozilla 1.7.12

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. It only scans individual files on demand. You can upload a single suspicious file which will be scanned by around 30 well-known virus scanners.