Press left alt, left shift and print screen at the same time. then press Ok on the resulting window dialog. It turns on high contrast mode. Again turns it back.
This one is just evil, set their IE homepage to /con/con to bluescreen their PC when they open IE.
Take the task bar and move it to the right side of the screen. Now grab the left edge of it and shrink it down to a thin line. Since you can still see the thin line, make a minor adjustment to the monitor so it’s off the screen. Now even a more computer savvy person will have a hard time finding it. What’s also funny is even if they push the Windows key or Ctrl+Esc, the start menu will show up in the upper left corner of the screen. This makes people who couldn’t find their task bar on the bottom now think it was moved to the top.