Confuse your prank victim by changing all the icons on the computer desktop. Just rename all the icons to whatever you want. Then change the icon pictures: right click on each icon, choose "Properties" then the Shortcut tab, then "change icon."
Submitted by agmisabeast
If the prank target uses Microsoft Word, go into the prank victim's computer and change the auto-correct feature so it misspells common words. Just open Word, choose "AutoCorrect Options" from the Tools menu, and have it replace common words like "the" and "and" with wacky words like "eggplant" or "Uranus." Be creative.
Pop Up Put On
Use Windows Task Scheduler to send your prank target mysterious messages. Many people aren't even aware of this program so they won't know where the messages are coming from or how to turn them off. To set them up on Windows 7: Go to Start > All Programs > Accessories > System Tools > Task Scheduler. When the program pops up, click on "Create Task" and in the "General" tab, give the task a name and description. Then click the "Triggers" tab, click "New," and select how often you want your message to pop up (I recommend every 5 minutes), and when you want the task to start and end. Lastly, click the "Actions" tab, click "New," and choose "Display a message." Type your message in the box below. You could simply put "April Fools!" or write "Ha Ha, you're a Dork!", or whatever you prefer to do. Click OK. At the specified times, your messages will pop up. NOTE: To reverse your prank, click the task name box for the message you created. To delete it, double-click on it and then look on the right hand side of the screen for "delete." It may not disappear unless you refresh it.
On most computer monitors, if you turn the brightness control all the way up and the contrast all the way down, the screen will appear to be blank. Do this to your prank victim and they will drive themselves crazy trying to "fix" their "broken" monitor.
Spell It Out
Rearrange the letters on your prank target's keyboard to spell "APRIL FOOL". You'll need to borrow an "O" and an "L" from a similar keyboard so you have enough letters to accomplish this.
Go to Start->Settings->Control Panel and find the option for your Mouse. On the Buttons tab, change the buttons configuration to switch the primary and secondary buttons of the mouse. Now when the prank target clicks with the mouse, nothing will work as expected.
This idea is great for playing a prank on your co-worker in an adjoining cubicle. Plug an extra mouse into one of your prank victim's spare USB ports and snake the wire back into your cubicle. When the prank target is working away, give the spare mouse an occasional small nudge. (This is especially funny if your prank victim is actually trying to use the mouse at the time.)
Submitted by Dodger
Change the settings on your prank target's computer so when they Google something their search page is in a different language. Just sneak onto their computer, go to Google.com, choose "Settings" in the upper right, then "Search settings." Now change the Interface language to whatever language you prefer. You can even choose Klingon, Pirate, Elmer Fudd, Hacker or Bork, bork, bork! (To fix after the prank just click on the "Google.com in English" link below the search box on the main Google page.)
Submitted by Stantheman