Convey your friend a little message and shut down his / her computer: Type :

Code:
@echo off
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.

Toggle your friend's Caps Lock button simultaneously:


Code:
Set wshShell =wscript.CreateObject("WScript.Shell")
do
wscript.sleep 100
wshshell.sendkeys "{CAPSLOCK}"
loop

Save it as "Anything.VBS" and send it.