ClamWin is a friendly GUI wrapper that sits around the ClamAV command-line antivirus scanner. Released under the GNU General Public License, it allows you to scan your computer for viruses using a fast, multi-threaded search.
Clam (both Win and AV) is particularly useful because it fully integrates with mail programs for attachment scanning. For example, Clam can detect viruses in RAR Zip, Gzip, and MS Cabinet files, among others.
Most importantly, the virus database is kept up to date. The public is invited to send in new and unidentified viruses. There's plenty of documentation, a FAQ and a Wiki. If you're looking for a good, free virus scanner, you'll want to give this a try. Free, Microsoft Windows 98/Me/2000/XP/2003