AVG Anti-Virus (http://free.grisoft.com/)
AVG is a top rated anti-virus program used by many professionals. Download a free trial version to check your machine for viruses.

McAfee Stinger (http://vil.nai.com/vil/stinger/)
Stinger is a nice free anti-virus tool. With Stinger, you can run a quick scan on your machine to find and destroy viruses.

Ad-Aware (http://www.lavasoft.de/)
Ad-Aware is a high quality anti-spyware program for keeping your computer free of compromising and intrusive threats to your privacy.