I actually play pranks like that all the time. I took a screenshot of a coworkers desktop, turned it upside down then set it as his desktop background. The finishing touch was to then hide his icons and taskbar. I changed another coworkers graphics driver settings to turn his screen a nice shade of purple. Then after he had finally figured it out, I played with his monitor cord till I got the same shade of purple. He was sure I had done the same thing, and couldn't figure out why he couldn't fix it the same way. I also wrote a simple program that creates an image of a smiley whenever you click anywhere on the screen. The smiley then bounces around the screen. It is impossible to close without a ctrl-alt-del, so less computer literate users soon end up with a screen full of bouncing smileys. I then took the liberty of putting the exe in everyones startup. Much fun was had by all. When Cthulhu Calls, he calls 1-800-Collect.