Unless you upgraded and saved your old OS you will have to format and start fresh to go backwards in the windows timeline. If you want to save you files and they aren't encrypted through NTFS then you can also just wipe your windows folder from dos and should be able to re-install without loosing everything. All your registry entries for games and programs would be gone though so if they're not re-established when you run the program again then you may have to re-install them if something doesn't work quite right.