I have 3 possible solutions I can think of...
-if damage is minimal-
1. Insert your OS Disc and select system restore and restore to an earlier time, before you messed with it.(usually the quickest fix.)
-if damage is moderate-
2. Try to run in safe-mode, and get all the important data you can, such as photos, drivers, etc., then do a fresh install of your OS. (recommended.)
-if damage is severe-
3. Insert your OS Disc and select install, and be sure to format the hardrive of all files, and do a fresh reinstall, (LAST RESORT OPTION, YOU WILL LOSE YOUR DATA!)