Put tape over the optical sensor of someone's mouse.
Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.
Install the Blue Screen of Death screen-saver on someone's computer.
Pull on a coworker is to press ctrl+print screen on their workstation, then paste it into Paint, save the pic, and set it as the desktop background. Move all of their icons to the trash. When they get back to their desk, clicking won't accomplish anything!
Chuck Norris can open PDF files with Microsoft Excel.
Fill someone's hair-dryer with baby powder.
Hide an alarm clock in someone's bedroom and set it for 3:00 a.m.
I’ve invented a human computer. When he does a mistake he blames another computer.
Windows, the world's first commercially successful virus!
Write a message on an upside-down paper cup that alludes to something horrible being trapped under it. Leave it on a coworker's desk or in a conference room.