As already said s-video is a garbage connection for a hdtv Flappy Frog is correct with the vga to dvi you should be able to connect to your pc graphics card easily and retain the hd quality your looking for.
Also connection to cable tv and using a hdtv you do not need a special converter. Just plug the coaxial like directly into the tv set not into a vcr,dvd player/burner, or any other device and run a channel set up on the tv, this should find all the avalible channels including the free hd ones.
Coaxial can carry hd quality but, plugging in another device and not running straight through the tv will downgrade the image quality. Unless that device is a upscale dvd player/burner, you will only get a certain amount of hd programs (at least where i live i belive there are currently 16 free hd channels). To get more channels you will have to pay for a hdtv box from the cable company however it is not a converter box only a box to unlock the signal codeing.
If you do not want a cable box sitting in your entertainment display and your tv set offers the feature there is something called a smart card that will plug directly into the back of most new hdtv sets this card will act like a cable box in decoding the signal but the card also has to be rented from the cable company.
Like i said this is all based on what im running and where im located your area may not provide you with any hd channels for free, you should call your cable company to find out what is already available in your area. And another thing that depends on your area is, some companys claim that 720 is hd other say only 1080 is hd if the program is broadcasted in 720 then it should look the same quality as a dvd would look on the tv at 720.
http://www.hdbeat.com/2006/02/08/how-to-connect-your-pc-to-your-hdtv/
