How about this one?
I waited for a co-worker to leave his desk one day. While he was in the men's room I went to his Win2k desktop PC and hit the "Print Screen" key. I then opened Paint and hit ctrl-v and created a bitmap file. I made this bitmap image his desktop wall paper and then dragged all of his desktop icons off of the screen.
When he returned, double clicking the icons produced no action. We all watched as he tried everything he could think of. I had to step in though as he picked up the phone to call the IT dept. No sense getting them on your bad side.
Just last week I went to the same guy's PC and opened his Intel Extreme Graphics GUI from Control Panel and set the display to refresh at 90 degrees rotated to the left. That one pi$$ed him off.
I noted the screensaver my boss was using, renamed this one to match, then mapped to his NT box and overwrote his old screensaver. When he kicked into screensaver mode, it put up the BSOD (complete with flickering busy light), checking files and finding eveything corrupt. He'd hit reset, and everything would be fine until the next time his screensaver would kick in. It was a hoot!
We had a gal who was very computer literate. We spent more than a few minutes getting our computers reprogrammed to english... geesh...