If I were you, once you re-install everything (you asid you had 2-80gb) in C, GHOST DRIVE TO DRIVE to D then just connect C and leave D in the case UNPLUG, so when he screws up, all you need to do is waste 1/2 hour installing everything again, better that hours and hours and reboot and reboot etc...
If you leave the D plug in or have an image of C in the system that he can access, he could accidentally delete it.
If you are using 98, then at least you can create a folder with ALT255 and put the image there and when that folder is double click within 98, it won't open, nox XP can open it no problem. This is how I used to hide PRON in my system from the kids so they can't access it unless they know COMMAND PROMP DOS COMMANDS.
I'm sure there is a program out there that let you preotect a FOLDER but i never look into it, maybe someone here could refer one so I could try it. I missed my PRON cause now I always delete it after view
