Poledit is reasonable if there’s no way to get a better OS on the machines, but it’s not totally secure if someone really wants to mess with the machine.
Might be an idea to get something like Ghost to take an image of a clean machine that you can drop on if things get messed up.
Ver-sys makes a product that keeps a snapshot of your hard disk on a hidden partition, and can restore it on a moments notice. You can do anything to the computer, introduce a virus, delete key files, or even delete the whole hard disk, then reboot and it’ll come up fine. They sell mainly to larger organizations; I don’t know what their minimum quantity sale is, but they may be worth checking out. If you do, tell them Bill Hunt sent you. www.ver-sys.com
Even better for lab use: Centurion Guard. This is a hardware device that write protects your hard drive. It puts all changes into a temporary file. When users reboot, you get a clean installation all over again.
We use it on a bunch of lab machines, and it’s great. The only disadvantage is that it has to be turned off each time you add software, but that’s minor compared to all the advantages.
Users can do anything they want and it’s fixed on reboot. Even if the reformat the hard drive, it’ll be fine.