johanneskuehne -
VGA and s-video signals are different in the way they "compose" a signal. You may be able to set your video display to duplicate, but you may not get the quality you expect relative to VGA monitor.
You will probably get better results on the s-video (TV) if it is selected to be the only output when watching a movie.
If you TV has a VGA, DVI port you will get better results. Many video cards also have HDMI and are a good choice if your TV has that feature as well.
Good luck and post your updated comments.