I may have some of this inaccurate, but you'll get the idea. With XP, the verification process takes a snapshot of your system when you verify authenticity. When you change out major components, hard drive for example, it appears as if the OS is being loaded a second time on a different system. I believe all you have to do is contact MS (sounds easy), and tell them that you had a hardware failure and need to reverify or something similar. They should then give you a code/key to allow the reinstall. It is a pain in the arse, for sure.
