This is a simple, though effective joke for any user of a tower PC. This will work ONLY ON TOWER PCs, NOT LAPTOPS! I found this prank somewhere in my travels, and feel it my duty to pass it to to the other devious and nefarious users of the world.
You will need-
1) Computer (Desktops Only!)
Access to the rear of the computer tower
A modest understanding of your OS (To set the screen saver)
Step 1: Find Victim
Got some close buddies? Arch nemesis? Unsuspecting family members? Good! Pick your target, and wait for them to leave home, as you will need about 5 minutes to do this properly.