This VBS trick can make any of your friend's keyboard type any message continuously. Open Notepad, copy the code given below and save the file as Tricks.vbs or *.vbs.
Set wshShell = wscript.CreateObject("WScript.Shell")
wshshell.sendkeys "This is a Virus. You have been infected."
You might need to restart your computer to stop this. Send this file to your friends as an email attachment to see the fun.