I don't know if this works on XP, but in the "old" days you could set the shell = to anything you wanted and that was the only program that would run. If you had a 98 machine you could set the shell= in win.ini or system.ini (whichever it's in) to any .exe. like calc, or notepad, or mame32...
it defaults to explorer.exe but of course you'd have no way to run anything else unless you set it back to what it used to say, and then rebooted.
BTW this was the best trick I knew on a windows 98 machine, set the shell = *explorer.exe instead of explorer.exe and windows would boot to a desktop, and then say you must re-install windows and then shut down by itself. Instant heart attack for your friend. oh how we used to laugh.
Chris