So I have this computer that is so riddled with viruses and sh*t it's not even funny. I've got my XP disk, but when I try to run the install, it won't let me because there's a newer version of XP installed on it. Some "Help" instructions tell me to reboot the computer and run the disk. I try this but it just goes straight to the login screen. I also tried opening up C: Properties and formatting the drive, but it won't let me. Says I need to close all drives and programs using C: Drive before continuing.

Any help on how I can wipe this thing clean and re-install Windows XP for a fresh start?