Here's a quick little practical joke that you can do to your friend, (or computer illiterate enemy)
close all windows and everything on the screen, so all that is seen is the icons on the desktop. move the mouse off the screen, and hit 'print screen'. open paint, and paste the image, then save it. set the image as the wallpaper, then right-click on the desktop, 'Arrange icons By' and uncheck 'Show Desktop items.'
if you did it right, you shouldn't see any difference. but it just removed his icons from the desktop, leaving the wallpaper you just put there. that looks exactly the same.
don't worry, it's easily fixed the same way it was done, but if done without the subject's knowledge, it can drive them mad, until you walk in and fix their problem in about two seconds.