The installer has been updated to allow the setup files to be extracted without installing. If the GUI fails to initialize (though that should now be very rare), this function is invoked automatically. Otherwise, there is a link in the bottom-right corner of the GUI at the beginning of setup. This can also be accessed via the command line switch /E.
Later versions of the installer include a link in the bottom-right corner to extract setup files without installing. If this function is present, the /E switch can be used to invoke it from the command line. For example:AutoHotkey110903_Install.exe /D=F:\AutoHotkey /E
Source: Installer Options