Control Panel, User Accounts.
But you must have Administrator rights in order to change the status of a User's Account. Are you the only user? If so, might be a problem. But try it and see what happens. If there's another user with Administrator rights, log on as that user (assuming you can i.e. know password, if needed) and then change your account.
Or - try repairing XP using System Restore (but this might restore the problem you've just eradicated - sorry!)
Post back results, please