ATI Radeon x1400 problems - theatre mode

A key feature that I used to use on my inspiron 9400 (until upgrading to vista) was to play videos on a TV externally using the s-video out. I had Windows XP media centre before and whenever you opened a video on the desktop it would automatically play fullscreen on an external monitor.

 

Since upgrading to Vista, I have not been able to do this even though all my settings look like they are correct... Any help would be much appreciated. It's quite urgent as every week I run videos in a live entertainment show from my laptop under these settings.