I think you just want to watch Astro on your PC's monitor,right?Than you'll need to get an analog RCA,multi input convertor box to VGA/HDMI/DVI output.Some older Ati card has S-video in/out,then just get a s-video cable to connect Astro decoder to monitor.
If you have Astro B-yond (lucky bugger) then just use HDMI cable lor.