XP Pro can only be activated on one PC at a time, so it won't let you load it onto a second PC when it's already registered on a first. If you still have it loaded on another PC and try to fool M$ into letting you re-register it onto a second PC that would be illegal, and thus outside of TIMO's discussion boundaries.
If the copy you have is not currently registered and installed on another PC, then you could load it on the "new" PC and call M$ to discuss why you are registering it on a diff PC.
You will need to do this by phone, as the automatic activation won't work if that copy is already registered in M$'s database, I think.
Cheers
Mick