The best thing to do (which i do), when the machine is in a perfect running state i run Norton Ghost to back up my brother in-laws machine to cd-r`s. so he can do what the hell he wants online and i can get it back to the way it was before he ever touched it :-)
If they are total newbies i recommend something like Norton GoBack which can be set to revert the machine back to the way it was at a set date/time.
