If you have USB, try PS/2, or if you have PS/2, try USB keyboard. You could also try resetting CMOS with reset jumper, as settings corrupted, may have disabled on board components (network,sound etc) which would explain XP wanting to activate.
Did you install any drivers recently? There was an article on this site recently saying how XP would ask to be reactivated if you installed certain drivers (like hard disk controller drivers) that were much more advanced/different than the ones Microsoft supplies from a fresh install.
Did this problem occur out of the blue or were there symptoms before?