Have you formatted your computer already? I have read it somewhere, (can't remember where) that you can swap the hard disk from one computer to the other although I was not able to confirm this myself because I have yet to experience this situation.
Since you are going to format your computer, can you try to boot in safe mode and remove/uninstall the disk drives entries. Also remove the IDE/ATA/ATAPI (if it's possible, again I have no first hand experience on this). Shutdown the computer and transfer it to the other machine.
The guy explained that swapping hard disk wont work if the disk and controller are still there because its tied to the mobo and if you remove it, XP will re-install this on reboot, and that reboot will be in the other computer.
Hoping that you have not yet formatted and will hopefully enlighten us if this method does work or not.