Format your harddrive, reinstall windows and setup all drivers without beeing connected to the internet.
Then use some disk imaging utility like
http://www.drivesnapshot.de/en/index.htm
and store a compressed copy of your drive in a safe location. It only uses like 2-3gb for a fresh windows installation, even an USB stick will do.
It's just simple as that: When a virus has managed to penetrate your firewall/defense, load your drive snapshot and try again!
You loose perhaps half an hour, but only if some virus manages to get in, and it's better to have a working system than sitting on a "cleaned up" installation, not knowing for sure if all remains have been wiped out!