I have avira. Its pretty decent. Doesn't eat up computer resources like most anti virus programs do. I use with with avast. Also try the free Maleware Antibytes program. It takes forever to scan your computer, like 2+ hours just for a small hard drive, but it finds alot of stuff others miss.
When I first saw the title of this post, I thought u meant STD. lol.
I have avira to, free and keeps your computer on normal speed. Specially with vista, just download it online and remove you old program before you start this one up. 2 virus programs on 1 computer means FIGHTCLUB inside your system.