If you are running XP, you should be able to do a 'system restore' relatively easily...
Go to the 'start' menu, then choose 'all programs', then 'accessories', then 'system tools', and choose 'system restore'. There should be an automated system restore point that you can revert to, and if you want to get it back to how it was from day one, backtrack through the restore points to the first one.
Chock