Here are links free and trial antivirus programs:
Grisoft AVG Free
. This is an installable free anti-virus program
Microsoft Live OneCare Safety Scanner
. Currently free, this is a web-based AV and diagnostic program.
Microsoft Live OneCare - 90 day trial version
. Installable version of above.
McAfee - 30 day trial version
Symantec Norton Anti-Virus - 30 day trial version