So you've never reformatted it? All you're doing is backing up your files, not the OS. You then just put in the Windows CD, reformat, reinstall, and... oh, that's right, you have a prebuilt.... Your existing Windows CD may not work anyway if it's Sony OEM stuff. Copies like that often will only install if they detect hardware from that OEM. If you need to buy another copy, just go on Newegg and buy the OEM version with a screw or something (to make the purchase legal).