first off, see what s-video from the pc looks like. it wont do the high res (tv-out never does more than 1024) so whack it down to 800x600 and have a play, you should be able to display a load of colours etc to check its all working properly. if you cant get s-video to wrok from the pc, you're wasting your time setting it up for the freeview box.
incidentally, divx dvd players can work quite well