You can't reformat the HDD within Windows - not even by using the DOS prompt. The DOS prompt in Windows is actually a DOS emulator and you cannot format the HDD with it because it's in use (Windows is loaded up from it). To reformat from DOS, you need to start the computer with a bootdisk (click here) and go into the DOS prompt from there. From there, type in FDISK to run the Fixed Disk utility. Delete all partitons and make a new primary one. Restart the computer with your bootdisk then type in FORMAT C: and this will wipe the drive. Boot the computer with the Windows XP CD and install a clean copy. Done!