Yeah, in the end i just gave them a ring and got a code for it, then reinstalled the legal software afterwards.
I didn't mean to accuse you of lying, but it would be easier TO lie, and just say you've changed the computer's hard drive and that you need to reactivate the computer. Then carry on using your son's version of XP.
OR you can get a code/program that deactivates the activation bit of XP, i've no idea how to get it but it exists. There's nothing wrong with you using it if you're going to uninstall the copy of XP anyway... (it's a hangover from beta testing i think)
Someone here will know about it.