Free AntiVirus Utilities

PC Tools Antivirus  (Free) 

This vendor provides a free edition of its PC Tools Antivirus program that protects your system against viruses, worms and Trojans. It includes a real-time scanner that checks files that you run (etc). The program detects, quarantines (places infected files in a safe location where it won't infect anything), disinfects (removes virus infections) the malware. Regular updates are free and there is no time limit. (The paid version adds priority updates as well as support.) Also available on that page is ThreatFire Free Edition, an antivirus product that does not work by using signatures to detect malware (such as virus signatures), but operates by providing behavioral (behavioral) protection, meaning that it looks at a program's behavior to figure out whether it is a virus. This allows it to detect new (zero-day) viruses, that is, viruses that have just burst onto the scene and have not been analyzed by antivirus vendors yet.
 

AntiVir Personal Edition

AntiVir Personal Edition is a free antivirus program for Windows computers. It has a resident component that monitors file movements for signs of viruses. It also allows you to scan your computer and repair files that have been infected. The program comes with a facility to automatically check the AntiVir website for updates to the virus definitions and download them. The free version pops up an annoying banner each time it updates, and does not scan/remove spyware that the Premium version (commercial) does.
 

AVG Anti-Virus

The AVG Anti-Virus software comprises a resident program that works in the background, an email scanner (checks your email for viruses), an on-demand scanner (eg, you invoke it to scan your system), the ability to repair certain infected files, and automatic updates of the virus definitions
 

avast! Home Edition

avast! Home Edition is free for home users on non-commercial computers. You have to register your copy and supply them your email address so that they can send you a license key to activate the program.
 

Rising Antivirus

Rising Antivirus is a new antivirus software that guards against virus, trojans, worms and rootkits. Besides the usual resident scanner, it also checks your incoming and outgoing email (POP3/SMTP), watches out for hidden processes and malicious behavior, and has application access control and program startup control (probably meaning some sort of host intrusion prevention system).
 

Comodo Internet Security Suite

Comodo Internet Security Suite includes both a firewall and an antivirus program. The antivirus software component provides real-time scanning, where it resides in memory and checks your programs for viruses as they are run, as well as on-demand scanning for viruses. You can also schedule your scans. It automatically updates itself through the internet. This is a Windows program.
 

Clam AntiVirus

ClamAV is a free command line antivirus scanner for Unix systems. You can also integrate it with your mail server for scanning your email messages and attachments. It is able to scan inside archives like RAR archives, Zip archives, Gzip and Bzip2 archives (that is, it has unpacking engines for those types of compressed archives). If you are a programmer, you can even integrate the antivirus scanner into your own programs. This free antivirus program comes with source code that is released under the GNU General Public License. Prebuilt binary packages are available for numerous Linux distributions (Debian, RedHat / Fedora, PLD Linux, Mandrake), Microsoft Windows, FreeBSD, OpenBSD, AIX, OSF and Solaris 8 (Sparc).