What you need is something like Steady State (a Software for libraries Internet cafes and such) Which enables you to have a fresh clean PC after every reboot.
Unfortunately Steady State isn't working on se7en
But Microsoft published a paper how to set up Windows 7 to resemble the workings of Steady State as close as possible:
Download details: Creating a Steady State by Using Microsoft Technologies
The other way would be to set a restore point and have your Computers go back to that point every morning (it basically restores the complete registry and shouldn't take more than say 5 mins)
Registry - Backup and Restore