Good that I have a Mac.
I use avast! on my PC and never have any problems. Probably because I only use it for gaming and don't even touch a browser on it.
Anyways what I recommend for Windows user:
C: Windows + Basic Programms and Drivers (Don't put personal stuff here)
D: Games, Data, etc
Get Acronis True Image or similar, format and reinstall Windows "with" Acronis True Image. Then install all the basic programs like: All Drivers, AntiVirus (I used avast! and it was just fine) iTunes, VLC, MSN, Firefox, etc ...
Then after everything is installed make an image of your C drive using Acronis True Image. Burn it on a DVD or put it on a virus free external hdd.
Now you have a backup. Everytime you fuck something up you simply format your C drive using Acronis True Image to restore it to the dumb&virus-free state. All your data you need will still be on the D drive.
Heil Time Machine for MAC!