My mom has comcast and I have her on a replaytv. Recently Comcast turned off ALL analog signal in her area.. Each analog TV needs a comcast provided converter box. The ReplayTV is analog. It needs a converter box as well, and the appropriate code enabled in the ReplayTV's setup.
If you still have an analog TV see if that can pick up any stations from Comcast.
When comcast talks about registering a Tivo, I think they are referring to a Tivo with Cablecard. In that configuration, you don't need the comcast converter box.