Generally the S-video on a computers video card is an output(to connect the computer to a TV) and can not be used that way.
There used to be some cards with video in, but i do not think there is a GTS 250 like that.
If you need to get signal from a DVD player or other device to a computer screen, a TV capture card for the computer or a box that just converts the S-video(or composite with adapter) to VGA or DVI.
TBH a cheap tv works better(those boxes are not always cheap and a capture card can have delays).