http://www.faronics.com/en-uk/standard/deep-freeze_en-uk/
I've used this for some of my XP computers. Now XP might be old, but it's still dependable. Deep Freeze is a program that 'saves' a chosen partition(s) in your Hard Disk. Once you reset, any changes to that partition will be lost. It's a great way to keep your system from getting any serious damage. Viruses, Trojans and other malicious software will disappear once you turn the computer off.
But Deep Freeze does have a problem. You shouldn't change the partitions(add, resize, remove) while Deep Freeze is installed. Even in the Thawed State. It's too risky. Chances are, you'll end up having an irremovable Frozen state. Where uninstalling Deep Freeze will not remove it. Once you restart, it's still there.
This problem needed a reformat to deal with.
Another thing about that program... every other month it seems to corrupt the boot sectors. Easily fixable by using Super Grub Disk.
http://www.supergrubdisk.org/