The ol' Desktop Icon Switcheroo

In Microsoft Windows, if you press the 'Print Scrn' key on your keyboard during any application it will take an image of whatever is on screen and place it onto your clipboard. From there, you can paste it into whatever application you wish...

So here goes. When your co-worker is distracted shut down any applications they have running on their computer and capture an image of the desktop, with all of it's icons, using the Print Scrn key. Open paintbrush and click Edit-->Paste to paste the image into Paintbrush. Save the image as a bitmap, and return to their desktop. Right-click the desktop and set the Desktop wallpaper to the image you just saved. If you wanna get real mean... delete all of their desktop icons (Might get you in trouble with IT).... The icons will still appear to be there, but since they're only part of the image and not really icons none of them will work. Endless hours of fun as they try to figure out what happened.

Bonus points for pulling this trick on a compter saavy person. No-one ever figures out whats going on with their computer.

* I did this where I used to work...I sat back and laughed my ass off, the other technician came over and he got so pissed he unplugged it!