This is a good prank that i heard about at school the other day...
First, right click on the clock in the right hand corner of the screen, go to properties and turn the clock off.
Then go to the desktop and close all the windows. Then take a screen shot of the desktop (CTRL-Prt Scr).
Then paste it into word pad or paint and save it as a *.bmp.
Then copy all of their icons to somewhere else on the hard drive. If they have XP turn all the icons off by right-clicking, then left-click the properties option, then go to desktop and uncheck all the icons.
While you are in the Desktop properties change the wallpaper to the screen shot of the desktop with all the icons.
After clicking ok on all those windows right click on the taskbar and go to properties, click the autohide option. Then to have a little more fun drag the taskbar to the right hand side of the screen.
Now you should be set, just leave the mouse pointer in the middle of the screen. The screenshot you made will have what looks like the taskbar on the bottom and itll look like their icons are still there.
Sit back and watch as they try to click on the icons and restart their computer. after that doesnt work you can be nice and restore everything or you can be really evil and let them call tech support!