Removing Admin Rights From User Account?

My step son downloaded a bunch of virus' which hosed my computer. I just finished realoading windows and all my software and now I'm going through and removing things from his start menu so he only has access to limited programs, he's 9 and he doesn't know to go into the c:/program files to get anything. I was in his standard user account moving stuff so it would only show up on my login. So when it came up and asked for an administrator password I typed in my password and did what I had to do. The only problem is now he still has access to those things that I authorized. I can now go into his account and access my documents. I would think you would have to put in the administrator password every time but obviously that's not the case. How do I reset that so he no longer has access to that stuff?