Are you sure your video card has an S-Video cable? My 7800GT, which has VIVO, does not have an S-Video port at the back; it actually has a special port (that I don't know the name of, or even if it is standard) that looks just like an S-Video port but has a different arrangement. A dongle plugs into this port which has many other connections on it, including S-Video In, S-Video Out, Composite In, and such. Do you not have a dongle that came with your card?
Notice that card has a dongle supplied with it. Some of them don't, though. Can you find your card on Newegg? If you can, you might want to check and see if it is supplied with a dongle.