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) Victim
1) Computer (Desktops Only!)
Internet access
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.

Step 2: Find Image

Go to your preferred search engine, and type in something like "Cracked LCD Screen" in image search. Find an image that fits the resolution of the monitor, and download it. Put it in it's on folder, free of ANY other files. Yes, that's right, a folder with just one image in it! Shocking, I know.

Step 3: Set Image As Screen Saver

Depending on the OS, different systems will have various steps for setting a screen saver- For Vista, which I use, right click, then go to-

Personalize>Screen Saver,
Click on the drop down bar, select "Pictures, select "Use Pictures and Videos From...",
Browse the folder with the LCD Screen image in it,
and click "OK".

ALSO---- Make sure the time before the screen saver turns on is set to the absolute lowest, say 30 seconds. If your PC will go into it's screen-saving state before then , make it do so!

Step 4: Unplug Mouse and Keyboard

Go behind your target's computer, and follow the wires from the mouse, keyboard, and any other peripheral besides the monitor back to their connection points. Unplug all of them , but leave them close enough to the tower that they appear to be connected.

Step 5: Sit, Wait, and Try and Keep a Straight Face.

All the preparations are now done, and you have no challenge left but to keep a straight face as you enjoy the torment of your victim as he/she struggles to discover the cause of this tragic incident. If you want, feel free to augment the joke with a believable story about how you "Accidentally" "Broke" their Screen.

Thanks for reading!

PS, I want to hear your stories! Let me know who you targeted, why you pranked them , and how long you went before you decided to be a merciful over lord and let them in on this insidious prank.

Have fun, and enjoy! ;D