XP has been around long enough for you to know and try SAFE MODE. If you have forgotten, reseach XP SAFE MODE and after that System Restore to the day before the failure.
My apologies for posting this again, but it has been some time since this last happened. My friends PC will not boot up. It goes into a continuous re-boot, but it doesn't stop at the error screen long enough to see what the problem is. It flashes for a second and then re-boots. This is a home built computer running windows XP.