Download the "Blue Screen of Death Screen Saver" and put it on a co-worker or friend's computer, and get ready for them to freak out every time the screen saver pops up. for best results, make their screen saver run after 1 minute :) - download the screen saver and find the instructions here.

Another trick that J-izzle pulled on me today is to switch the mouse button functions on a victim's computer. Details to do that are here.