Ending and Exiting your Installation

A very simple question, but I can't find the answer! :-(

How can you exit the installation from any part of the installation, is there a command to enable me to do this?

an example where I want to use it:

StrCmp $R0 'true' okay notokay
notokay:
MessageBox MB_OK "Error: You don't have Administrator rights to install"
CloseDownInstallation

okay:

Quit command.

:-)