I think that the only way you could do this is with a video capture card or a TV card.
You would have to install a new card (cheapest about £35) that would allow s-video input into your pc. I would then use a peice of freeware call dScaler to show the image on your pc.
I can recommend the Pinnacle PCTV Rave card - I bought it from maplins for about £35. If you use dScaler you don't need to install any drivers or other software. dScaler will automatically recognise the card.
It sounds as thought you PC might be oldish - make sure it is at least a Pentium 450 to be able to run dScaler full screen without dropping frames.
Aside from this there may be ways to convert s-video to rgbhv - have a look on
www.keene.co.uk
Hope that helps
Alex