(I take no responsibility for the outcome of any of these. You're on your own. I don't endorse any of you doing these things but it's funny thinking about doing these office "pranks" to that special someone in the office.)

1. Go into MS Word or similar program on co-worker's computer, and add an entry to the AutoCorrect feature. This is a very simple prank that will send the novice user into frenzy. Configure the AutoCorrect option to replace the word "the" with the phrase "You're an idiot." They will usually panic and start scanning for viruses.

2. Take clear tape and tape the underside of the mouse. Make sure you take the sticky end of the tape and apply it to the bottom of the mouse so it locks the ball in place. The victim will most likely check the connections in the back, reinstall drivers, reboot, etc. before they realize what has happened.

3. Another gem is to do a "Print Screen" of the user's desktop, and then paste the image from the clipboard to a photo program, and save the image as a bitmap. Then, set the 'snapshot' of their desktop as the actual desktop wallpaper. The user will see their desktop as always, but everything on it will appear to be frozen when they try to click on it, sending them into a rebooting and virus scanning fit.

4. This will mostly only work with people with very little PC knowledge. Stick in a floppy in their floppy drive. They will be unable to boot up windows until the disk is out. This is fun to watch.

5. Try to find a very obnoxious CD laying around. Pop it in their CD ROM. Put up the sound full blast by double clicking on the volume control on the bottom right. On normal configurations the audio CD will auto-play when windows first starts up. The person starting up their PC in the morning will definitely be embarrassed.

6. This is for that special person you just can't stand in the office, the one who talks on the phone all day with their boyfriend/girlfriend and gets personal email all day. Go into their email and change their defaults to automatically blind copy their boss. Heads will roll.

7. Change the coffee in the office coffee maker to decaf. Wait about three weeks and switch to espresso.

8. Try password securing someone's screen saver. First I suggest changing the screen saver to scrolling marquee and inserting your own word or phrase, "Mr. Jones (president or supervisor) eats beans" or something to that effect.

9. Pop out the "m" and "n" key on someone's keyboard and reverse the two.  Any flat tool will work. Just pry it with little pressure and they will easily come right off. Then just sit back and watch the confusion.