This trick is rather old but you can easily play this joke on a beginner, but sometimes even experienced computer user can be tricked.

So, the idea is: when a tricked person leaves a computer, you open any application on his computer, for example "Windows Media Player" or a folder but not for the whole screen, somewhere in the center of the computer. Press the key "Print Screen" on the keyboard and close application. Then open "Paint" program and choose "paste". This screenshot will be displayed in Paint. Save it somewhere in C:WINDOWS, give it name, and choose "Set as desktop background". Close Paint, so you see the image of his desktop with the open window in the center instead of his desktop background.

When your victim sees it, he/she will try to close the window (of course, they will not be able to do this) and at least they will restart the computer saying several unquotable words. Can you imagine their astonishment when this window will appear again?