Prevent User from Installing programs

I have some laptops set up with Windows 2000 and the user was set up with “Restricted User” access but this caused a few problems so I am trying to set the User up with “Power User” privileges but I want to be able to prevent the user from installing any programs like what happens with “Restricted User”.
I have set them up to prevent them from installing programs with Windows Installer, but a few programs will still install with this setting if they don’t use windows installer.
Is there something else I’m missing?