Do you mean the activation code? Presumably if you've reinstalled XP you're past the licence number bit that I think Spin's talking about? If your original set-up came with XP pre-installed, the licence number should be on a sticker somewhere on the base unit.
I don't think there's a record of the activation code anywhere, you just activate the new install, either on line or by phone. I reinstalled XP a number of times, with and without new hardware and had to do the activation by phone. The phone option goes through an automated entry system first and if that fails you get a real person. They never queried why and always gave a number which worked without any snags.
Any of that help?