I don't know the answer but I think if everything is the same (even computer name), you won't need to deactivate-reactivate. But who knows.
But, anyway, forget totally about XP and go to Vista or 7 64 bit, it will give you a big boost on fps because it will use up to 2 cores of your cpu. XP will use just 1. Also, you won't need to reformat every x months anymore, perfomance will be the same (neither you need it in XP, except if you put a lot of crap and in that case, you another testing partition/OS...)
Really.