Help with locking windows xp...

I have a little brother, and we're both gamers. However, my mom wants to limit his ~10 hours a day of gaming so that he can get his homework done (though I doubt it will work). My solution was to simply lock XP in my account (since we each have accounts, and because we game they're both administrators). How can you lock XP so that when you go to unlock it, you can't change the user name and login under another admin account? I know there's programs that can do this, but I know windows can do it since I've seen it done at my previous school. Can anyone help?

I don't see why gaming requires you to be an admin. You should be able to lower the privs on his account and disable that accounts ability to change U&P. You might have to fiddle with the security policy, or add one if it doesn't exist.