Exactly what upgrader said. XP takes a snapshot (of sorts) of the machine it's on, and and major changes, such as a chipset or motherboard, and xp will look until you enter your key OR call MS. I've had to do this probably 30 times, they are always very accomodating, just explain to them that you moved your hd from one case to the next, and they'll hook you up.
And yes, keycrackers are illegal here.
