NOTE: You must register this program even though it's free. The registration will be good for a year. After that, you must re-register. It is still completely free though.
A great antivirus program that auto updates the definitions and optionally the program itself. Has a VRDB (Virus Recovery Database) that you set to help in recovering from a virus if infected. It also includes anti-spyware and anti-rootkit protections. Full of other features.