You boot up the computer with the windows xp cd in the drive. You then skip over the first screen where it asks you if you want to go into repair console and proceed like a normal install. It'll detect that you already have a copy …
Can you perform a repair install on the system? That's the only thing that can fix it at this point. Or a reformat.
What exactly is that and how would I do it? I'm not that great with terms sorry.