Do you have your original disks? It will take a few hours, but you can just reload everything to the day you bought the computer. Then, it will take a few days/weeks for all the automatic updates to go through (6 YEARS of them!)

I had to use the restore disks on my tower this past summer, I messed it up big time.

MAKE SURE you copy any photos, Word and Excel files and the like first as they will be lost.