Step1
Keep an eye on your target. When your friend is away from the computer for at least five minutes, it is time to make your move.
Step2
Look for open applications on the desktop. If there are no open applications, open one of the folders such as the "Documents" folder.
Step3
Move the computer mouse arrow down to the extreme bottom right of the screen so you can no longer see the arrow.
Step4
Take a screen shot of the entire screen by pressing the "Print Screen" button. This button is usually located near the "Delete," "Home" and "End" buttons.
Step5
Open an image editing program such as Paint or Photoshop.
Step6
Once the selected program is open, press "Control" and "V" to paste the screen shot into the program.
Step7
Save the image in a place where you can find it with no problem such as the "My Pictures" folder. Rename the file "Replacement desktop.jpg."
Step8
Once you have re-saved the image into a folder, close the image program.
Step9
Close any open programs that running from the main desktop.
Step10
Press the right click button of your mouse on the desktop. Choose "Properties" from the menu that pops up on your screen.
Step11
Click on the "Desktop" tab at the top. Click on the "Browse" button half-way down the box.
Step12
Find the “Replacement desktop” image you saved in the My Pictures folder and open that image.
Step13
Click the "Apply" button and the screen shot image should now be the background on the computer.
Step14
Click the "Ok" button to make the "Properties" box disappear.
Step15
Leave the scene of the prank.
Step16
The victim will find out soon enough that they cannot interact with any of the programs that appear to be open on the desktop. This will be a source of frustration and confusion for them.
Step17
After a few minutes of savoring the victory, wander over and see if you can help them with their problem. Have your friend describe the issue to you, then casually change the desktop background to the original background.
Step18
Reveal your brilliant April Fool's Day prank.