Free software to lock son's PC

I've done some searching, but can't seem to find anything. Basically I'd like to have software that will show an image on his screen and prevent him from using his computer, so he doesn't spend too much time on it. Is there any free program to do this?

Could you not just use a password that you know and he needs to have you sign in for him,if you are administrator & his account is limited you can stop him doing all kinds of things. But if you just want to lock him out put a password only you know,