Windows XP Home Edition will not start

Windows XP Home Edition would not start after installing MS security updates
recently. I understand this may be an issue with the product key used by
Gateway when the system was originally built. I have reinstalled XP. Is
there a way to change the product key from the generic key used by Gateway to
the one that is on the sticker on the side of the PC, so this problem won't
reoccur later when updates are installed?