It's probably stating the obvious, but it seems that the 3 "missing" pins are the RGB information, while the first four are signal and luminance. What does the graphics driver program have to say about all this? It usually blows up a "TV configuration" dialog box when the S-Video port is sensed as active. If not right click on the desktop, then choose "Graphics Options > Properties > Then root around til you find something that applies.
I know, I know, but I'm sitting at a machine with only single monitor capabilities at the moment, and I also don't know whose graphics driver you're using