Most HDTV Tvs will have multiple input jacks on the back of the TV. You may simply want to run a splitter for the Coaxial cable before it enters the Comcast box, and connect one Coax to Comcast Reciever, and the other to the TiVo. You will want to leave the connections as they are between the Comcast reciever and the TV, but add RCA jacks from the TiVo to the TV. Now, if you want to view something on the TiVo, you can select that input with your remote control, and if you want to watch HDTV programming, switch to the input which the Comcast box is connected to! I believe this is your best option.
I'm not sure on the capabilities of the TiVo series 2, but is may be able to recieve all of the digital channels as well as the standard analog channels. Comcast will be going ALL DIGITAL very soon, so the analog signal will eventually be turned off as it uses a lot of bandwidth to send an analog signal, as compared to digital.
Now, I didn't take time to read the information here, but TiVo has a very good solutions guide, here:
http://tivo.instancy.com/external/LaunchContent.aspx?cid=dcadf995-410e-407a-8e33-65a0ed6a14fa&anchor=undefined