Here are several different anti-virus programs. Some you may know, others may be new to you. Free Anti-Virus & Anti-Virus Trials. And as always, backup before you install.

My personal favorite. Offers a pre-boot scan, unlike most others. Has audio alerts and talks to you, to keep you informed.
Tough on viruses, easy on users, small program. High Ratings. I have been real happy with this product as well.
Norton 350, Internet Security, even protection for your Apple Mac.
Nice tool but you should not depend on this as protection.
Two free Mac antivirus apps compared.