This only happens on a physical PC if your hardware has changed significantly (e.g. change of mainboard or complete PC ...) or on a virtual machine if the MAC address of the primary Windows Network interface has changed.
Usually an activated XP/VS license is bound to the hardware of each machine.
However, based on manual requests we do allow reasonable reactivation requests.
e.g. if your hardware breaks, or you change your server, etc.
Please follow this detailed guide for migration/license transfer:
Additonal Links:crash, Invalid hardware key, move, no connection possible, reactivate, transfer license