And the Best Antivirus is...?

So my computer has caught a bad case of aids, the first time in 3 years. A good (at least) half of my memory is doing... something. Constantly.  I have no idea what. It's so bad that I can't even run videos in MPC without getting funky distortions in my audio while I have firefox open. I've been running BitDefender 8 (free edition) as my Antivirus for a while but that can't seem to get rid of it, so what do you guys think are the best programs to find out what the fuck is going on and kick it in the ass?