Dude the only thing that you can do is to back up the files that you want to keep. Just use an external hd or get a 2gb flash for like 15 bucks. Wipe it and then rescan the entire disk with your backups before you even start to load the stuff back on.

If you are familiar with what systems need to be running you can check them in your task manager and if you see something trace it. A lot of viruses have been found hiding in .dll's lately