oh that. I had to do that for dad. Take a look on something like KaZaAlite for xp keygen and activator.... If you want I can put it on the net.. Run the keygen, it will make you a good key, save it somewhere, run the activator (it will want to reboot) and put the key in.
NOTE: this may or may not be correct, I did this a long time ago.
also, this was originally on http://kickme.to/msbetang but now its being shutdown