Making Nero Showtime the default program for DVD movies

I’m running Windows Vista SP1. When I place a DVD movie into the the drive, Auto Play pops up and gives me a choice of opening the movie with either Windows Media Player, or Windows Media Center.

I would like to use Showtime to watch the movie.

When I go into the Control Panel > Auto Play, Showtime is not listed as an option.

I can play the movie if I first open Showtime, and then select the source of the film.

My version of Showtime is: