Install Deep Freeze Professional from www.deepfreezeusa.com
This allows users to install software, but when they reboot, it is gone. With a password you can unthaw the PC for the next restart, x number of restarts, or forever and install authorized software.
I did a test with a Win NT system where I installed Deep Freeze and then deleted the System32 folder, rebooted and it was back. Great for school labs.
