Deep Freeze

If you run a business and your staff keep accidentally getting their computers infected then we've found some software for you. This would also be suitable if you have a PC in a public area.
http://www.faronics.com/en-uk/standard/deep-freeze_en-uk/

Basically you start with a clean PC and apply the deep freeze software which stops changes from being made to the computer permanently. If something goes wrong you just reboot the PC and the original state comes back.