Scrambled user account?

I am running XP SP2 . I have two administrative user accounts and two limited user accounts. At themoment one of the admin accounts is failing to complete the logon process. The process seems to hang about half way through logging on. The other three accounts are functioning normally. I have tried using the system restore tool but this is not allowing me to restore to an earlier date. Has anyone gort any ideas about what might have gone wrong and, more importantly, how to restore the user account?

Many thanks in advance,

Coco349