How to unistall completely windows XP from computer to re=install a new version

I think the easiest way to do a clean install is to have Windows setup format the drive during installation. Any data to be retained would need backing up first. Otherwise, an upgrade install may as well be done if the current installation is healthy.

How to Format Drive and Install Windows from Scratch
Geek to Live: How to format your hard drive and install Windows XP from scratch (http://lifehacker.com/software/windows/geek-to-live-how-to-format-your-hard-drive-and-install-windows-xp-from-scratch-157578.php)

Why do you think you need to do this? If you do, you also need to get all your device drivers as well.

But if you explain why we may be able to suggest an alternative.