Well, assuming there's data you want to backup, you should do that first. Burn it to a CD, or whatever you need to do. Then, assuming you have a full version of XP, just stick the disc in the computer and reboot it. It will come up to the setup screen and when you choose "Install New" it will format the hard drive for you. It's all pretty easy to follow. Heck, even I can do it!
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