another solution would be to use the recovery console contained on the xp setup disc. it will automatically add any missing files etc for you. boot from the disc, ignore the first prompt to repair an installation pressing enter to go into xp setup. on the setup welcome screen, highlight the xp installation with the prob and press "R" it takes about the same length of time it takes to do a full install. most os probs can be solved using the repair console. it saves having to play around with registry settings and the like.
hope this helps.