Microsoft has (some time ago actually) released a utility (or security addon if you wish) for Windows XP called Steadystate;

What is does is basically to add very strict control to your desktop and to allow for guest or kiosk pc’s.
Lets say you administer a library pc, and daily have to deal with users changing settings downloading strange software/malware and basically just giving you a hard time.  Well with Steadystate you can inflict strict rules on what a user can and can not do, you can add time-limits for usage and finnaly (and this is the neat part) you can have the computer restore to a predefined setup at fixed intervals (or at every boot/logoff if you prefer).

In a corporate domain setup the software does not seem that interesting as many of the settings can be set via GPO’s, but for kiosk pc’s it’s neat.  And as the author of a commercial predecessor I should know 😉 did basically the same thing just not in the same dept and detail.

If you have a kiosk pc environment, hared pc’s or your kids just keep messing their installations up this may be worth a look.  and the final good news, ITS FREE!!