Here are some those pranks which you can apply to your friend's or officemate's computer.
Confuse your 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."
If the victim uses Microsoft Word, go into the 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.
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 victim and they will drive themselves crazy trying to "fix" their "broken" monitor.
Spell It Out
Rearrange the letters on your victim's keyboard to spell "APRIL FOOLS". You'll need to borrow an "O" and an "L" from a similar keyboard so you have enough letters to accomplish this.
As to April Fools Day origin, here's from Wikipedia.com entry.
In Chaucer's Canterbury Tales (1392), the "Nun's Priest's Tale" is set Syn March bigan thritty dayes and two. Modern scholars believe that there is a copying error in the extant manuscripts and that Chaucer actually wrote, Syn March was gon. Thus the passage originally meant 32 days after March, i.e. May 2, the anniversary of the engagement of King Richard II of England to Anne of Bohemia, which took place in 1381. However, readers apparently misunderstood this line to mean "32nd of March," i.e. 1st April. In Chaucer's tale, the vain cock Chauntecleer is tricked by a fox. You may continue reading here.