You can always try logging in to the computer in safe mode as the admin (an administrator login will show up on the welcome screen)...if your computer shipped with XP preinstalled then you should be able to just push enter when prompted for the password. From there you can change the properties of each account and turn the tables on your brother.
Short of that, I'd follow whizkids advice and kick his ass....as a matter of fact, I'd kick his ass even if I did get the problem solved just to make him think twice about messing with my computer next time!
