Just a suggestion, but you could try making GBPVR the shell so that GBPVR runs instead of explorer (desktop, start menu etc.) - all they would see is GBPVR, no way to start programs. Then apply a registry policy to disable task manager (Software\Microsoft\Windows\CurrentVersion\Policie s\System\DisableTaskMgr = 1) and they will have no way to start any other programs or access any files on the computer outside of GBPVR.

If you wanted to easily be able to escape this yourself, you could write a simple batch file and add it to the custom tasks that asks for a password, and runs explorer (like normal) allowing normal computer use, and then a second to kill explorer when you are done and want to go back to GBPVR only.

You could also use 2 seperate user accounts. A GBPVR user account with the GBPVR shell and locked down system, and a normal user, password protected. If its XP, you could even use fast user switching and switch between the accounts at the press of a button (and a password).

If I've misunderstood what you wanted, I apologise, just trying to help