As someone who has successfully ran 8 (although it was the CP) on an old system (P4), I question why you want to do this? Is it just because support ends? How much were you using this support before? XP is old, and it does suck in a lot of ways compared to Vista/7/8 but XP does run really well on anything made after about 1999, so if things are working for you now, I wouldn't screw with it.