In what way do you mean poor ?
If you are getting stuttering/mpeg squares then its either a signal level problem or maybe simply your PC isn't fast enough to copy with software Mpeg decoding.
If its some other problem like grainy picture or bad colour etc then chances are that your monitor is badly adjusted for video or you are using the wrong display mode (try getting the latest version of graphics drivers and DirectX - then set the various display modes - overlay etc.....). Another possibility is your graphics card. I've noticed some lower cost cards (especially Nvidia) seem to have limited colour rendition and always seem very dark !
LCD monitors also seem to be crap for displaying digital images. The refresh rate, even on so called fast LCD's, just doesn't seem upto action movies with blurring being very noticable. Even sub 12mS LCD's, which should cope fine with a 25fps movie seem to have this blurring problem - maybe its something to do with the way the image is built up onscreen and persistance-of-vision rather than refresh but I find it highly visible (I cant bear to watch an LCD TV for this very reason). I've also found LCD screens just dont seem to have the contrast necessary for vivid colours in movies.
Something else to watch is the native resolution of your display. Make sure that your display resolution is compatible with the resolution of the image you are trying to display. Stretching an image to your screen can make it appear very blocky and, depending on graphics card, can slow things down enormously.
When viewing, remember to take into account the screen size with your viewing distance. For a 17inch monitor displaying full screen video you really need to be at least 6 feet away rather than the normal 2 feet PC viewing distance. Too close and you will definately see blocks !
If your DVB card has a Video output then try connecting it to a standard TV. Check the quality on that and compare with a digibox. There shouldn't really be that much difference.