Trend Micro – http://free.antivirus.com/

Always a good free source if you think you have a virus.  Highlighted is their Housecall application.  Small app download that uses a smart scan technology.  Basically definitions sit in the cloud.  You can run a quick or full scan on an entire drive or select folders.  Also available is HijackThis application.  Great tool for the advanced tech.  Then there is RuBotted, CWShredder and RootkitBuster.  All great tools for any tech toolkit and the price is right.