You can easily do this stuff with a bootable partition manager.
You must login or register to view this content. is a good one. You have to burn it on a CD, and then you can boot it.
If the cd-drive isn't the primary boot station, restart your pc and press
del immediately, so you get into the bios. (some pc's require something like F10, just look at the bottom of the screen.
When you're in the bios main menu, use your arrow keys to go to "BIOS settings". Then go to "Boot Order" and change it with the + and - keys until cdrom is upfront. (Almost done
)
Now save & exit. Your pc will automatically restart. Be sure to have the new cd in the drive. Now CPM pops up.
Now you must
delete the partition(s). After this point, you won't be able to recover your files. Now make a new partition, and fill the entire disk with it.
There's always about 6mb of unpartitioned space left on a hard drive, don't worry about it.
You can now easily install a clean windows xp again.
If you need any help recovering files, before you've done this, just ask