Converting DVI to VGA is easy with an adapter. Converting VGA to s-video is easy if the video card supports s-video out through the VGA:
http://www.amazon.com/EFORCITY-VGA-S-Video-RCA-Adaptor/dp/B000P3UB24. If your video card doesn't support this (check the manual to be sure), it is not as straightforward and requires electronics such as these:
http://www.ramelectronics.net/audio-video/video-converters/vga-rgb-hv-to-s-video/c10000-c11200-c11214.html?sorter=productname-en-ascOtherwise, a card like this
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127301 would give you s-video with no conversion necessary.