NT tricks

I need help. I have a user who has been installing Aimster software ( file sharing program to download music from the Internet) on his machine and the software always messes up the machine and we always have to format his hard drive. This has happened twice. Now I want to make it impossible for him to install anything on his machine. I have installed NT Workstation on the machine. Is there something I can do to the machine to make it impossible to install anything on the machine without having administrative rights? Registry tweaks, script, or any trick.