The Advanced Upside-Down Desktop
This one is silly but satisfying. If his is the kind of PC that lets you flip the screen around upside down and sideways, you can implement a variation on old upside down wallpaper trick. It goes:

1. Take a screen shot of his desktop as normal. Rotate the picture 180 degrees and set it as his wall paper.

2. Remove desktop shortcuts and icons to a safe place or the bin, depending on how callous you are feeling.

3. Rotate the screen 180 degrees, using alt gr + cursor key or similar, so the wall paper and shortcuts will appear the correct way round.

4. For best results, also hide the Start bar

5. Retire to a safe place to watch the ensuing madness when they return. Those with VNC or RAdmin will be able to watch the sorry saga from a great vantage point. Watch as they struggle to control the mouse as it travels in the opposite direction. Laugh as they finally make it to an icon, for it to do sod all when they click on it.

Not embarrassing, but you say he's pretty dumb, so it'll take him ages to catch on.
(Cryptoprocta has abandoned the username of Voodoo Mary, Sun 27 Jan 2008, 10:43, Reply)