I had the same issues when I was installing the ATI HDTV Wonder card last week. It turned out to be my video card. ATI's published requirements for the video card are not as high as they actually need to be. I was getting all the channels in both ATI's DTV software and in MC but the 1080i and 720p channels were just a black screen with great sound. I had to upgrade the video card to 256MB (not 64mb like the specs say) to get the video to render. But then the video and sound was skipping not being able to keep up. THEN I had to swap around the cards in their PC slots so that the video card and the ATI HDTV card would not share their IRQ's with any other device. Then it worked. It is now working as of this weekend.
I am currently still using the ATI DTV software, though it stinks
, because it has the ability to record unattended. But I would rather move over fully to MC, since let's face it - it's the best media software known to man.