Here is the simple way to play prank to your friends.

1. View the desktop and press print screen button at your keyboard

2. Open paint program and paste it.

3. Save the image to any folder.

4. View the image as slide show.

Your friend see will see the slide show as the desktop. It would not have nay effect when he/she click on the program icon. Tested on my friend and it work like a charm.