I heard recently about a computer trick you could try on a friend. Of course, afterwards they might not be your friend any more.....
Go to their computer when they're out of the room, minimize any open windows, and press the PrintScreen button. This will save the entire desktop to the clipboard.
Open MS Paint, press Ctrl-V to paste, and save the resulting picture as a BMP file -- let's say you save it as "C:/deskfoto.bmp". Close the paint program.
Rightclick anywhere in the open desktop, and you'll get a menu that starts with "Arrange Icons by...". One of its choices is "Show Desktop Icons". Uncheck this to hide all their desktop icons. Then using the Display section of the Control Panel (see "settings' in the start menu), make note of their existing desktop picture (so you can restore things to their rightful place before your friend kills you), and replace their desktop picture with C:/deskfoto.bmp.
They will try to click on their usual icons, but since it's only a picture, nothing will happen. -- Rachel
P.S. Or you could leave their icons displayed, change their background as described, and .... MOVE a few of the real icons. The potential for mischief is high.