"When you activate Windows XP, Microsoft stores the data in the Windows Product Activation database files wpa.dbl and Wpa.bak in the folder %systemroot%system32. If you change the motherboard or make significant hardware changes, XP will require you to reactive. But if you plan to reinstall XP on the same hardware, you can back up the activation status and then restore it after you reinstall and avoid the activation process."

Hit the source page for all the details.