Convey your friend a little message and shut down his / her computer:
msg * I don't like you
shutdown -c "Error! You are too stupid!" -s
Save it as "Anything.BAT" in All Files and send it.
ALL OF THESE COMMANDS ARE TO BE TYPED IN NOTEPAD. Mail It Or Send Via Messenger. :) simple and easy :) Don't prank too much may end up in trouble or may become a wartime between you and your friend ;) ;p Enjoy!