mcody, just a few comments. It's unfortunate that your cable box does not have an s-video output, but the composite output will be better quality than what you are referring to as coax. The coax's signal is RF modulated to simulate a TV signal. There is usually some loss of quality in this modulation-demodulation process.
As for your computer, you will want to simply plug it's VGA output into the VGA input of the projector. The s-video output is not made to support the full resolution of a computer's video card and the quality of the output will not be very good. If you have more than two devices going to VGA, you can purchase a switching device, which can be expensive for a high quality one.