Windows XP/Vista only: SteadyState, a free Windows utility offered by Microsoft, is a handy tool to have if you've got kids, friends, or just a program or two that could possibly wreak havoc on your system.
Seems like a really cool app. I always thought I could "kid-proof" or "babysitter-proof" my machine by simply using a more restricted user in Windows, but this sounds much better. I'll probably try it sometime and see how it goes. I just hope it doesn't leave a large footprint on your machine.