I have a problem with a nforce chipset pc (built in nvidia mx400 gpu)
and the s-video out. It has done the following things (each one was
with a different version of the driver):

1) not work at all (MS version)
2) display black and white when I go to full screen while playing a
dvd in media player or WinDVD (MSI supplied driver)
3) always display b&w (most recent nvidia driver)

I am at my wits end. I have installed Tvtool and mucked around with
it, but got no positive results. What am I missing? The s-video cable
appears to be ok.

Is there an 'Output format' option?
I think you can get black and white if your output settings are 'NTSC'
but your equipment is PAL.  Ensure the output video standard matches
the equipment you have.
Also, make the the output is set to 'S-Video' and not composite.