Locking down the desktop…

I assist in the maintenance of several computers that are all used by grammar school students. Constantly they move around icons, change screen savers and colors, etc. Their user accounts are set to the lowest level of authority/access through Microsoft Windows (2000 professional). Is there a way to lock down the desktop or restore the computers on a regular basis to avoid having to manually fix them each time? Is there especially any way to do it without buying lockdown software?