If you dont have any encrypted files on your drive, then just pop in your windows CD and choose Repair Windows. It will write over your old windows but keep all your files. So you just have to get all your security updates again and reinstall your games (back up your saved game files first).
Simplest and most effective way.
Next time make sure you have auto updates turned on. Buy a good antivirus (Norton, F-Secure, or Nod32.. if you get norton make sure you get just the AV; their other system software sucks) (or if you dont have any money use Avira, it's the best free one. AVG sucks). Use a firewall (like Zone Alarm), make sure your router has NAT (& SPI if you have it) turned on.
Put a strong (as in word(s) & number(s)). A normal word can be cracked w/ a dictionary attack in like 5 mins. On your limited account and admin. Dont use your admin account unless you absolutely have to. (yes, at least 2 accounts. Admin & Limited.) When installing games, switch your limited account to Admin just for installation & first run of the game. Then switch it back to limited.
When you browse the net use Firefox, and run it in http://www.sandboxie.com (after you have set it up first).
Do this & follow these rules and you will be fine.