In the UK, where I live, we have a terrestrial channel, (not digital), called Channel 4, but since you mention Comcast, I assume that is not what you mean, and you are in the US.

So, I also assume that you would have had to tune your TV to a specific channel to receive digital broadcasts from your provider. That's normal. The TV would still receive terrestrial, over the airwaves, broadcasts for your other channels, but this channel 4 is specifically for digital broadcasts.

Back to your Media Center computer. You say you have 'lost' the ability to set a tape to record particular digital channels at specific times. Does that mean you were able to do this before on this computer? If so, what has changed since then? Do you get error messages, or, what happens when you try?