If you are doing it that often have you had a play with a mirror application.
Build the machine up, install all the applications. Do all the good stuff putting the data on a separate partitions and getting user accounts setup, pointing all the applications to this partition
Then mirror the whole lot. If they do then kill the machine USB pen boot into linux, backup data partition onto your USB drive.
Blow the image back onto the machine. Restore the data and you should be good for another 3 months.
Just keep all the mirrors on either DVD's or on a separate drive.
If they want to install some software blow the image back on again so its a clean install the software take another image and restore the data if required.
First time you do it it will take maybe a couple of hours to set up but after that you should be able to reset to clean in under 30mins (data backup/restore time excluded)