Formating your drive and reinstalling Windows causes more problems that it fixes. There is ALWAYS something you forget to backup, whether its drivers or data. But if you have to...I'm taking a wild(very wild, forgive if i'm wrong) guess here that you are trying to reinstall Windows while you are logged into Windows. Remember to restart your PC and boot from Windows Installation cd. It will prompt you to format your drive (If it is Win XP). If it is an older version of Windows, you might need boot diskettes to get the installation going...