From your description, I'm assuming you have the following setup:
Cable from wall -> Comcast HD receiver -> TiVo Series 2 -> HD TV
You cannot display a HD signal through a Series 2 TiVo. All Series 2 TiVos, with the exception of the DirecTiVo HR10-250, are Standard Definition (SD) and cannot handle the HD signal that you are trying to feed it from your cable box. You need to either get an SD capable cable box from Comcast to feed your Series 2 TiVo, or get a Series 3 TiVo and cable cards to properly record the HD signal.
In other words, in your present setup, you will never get a picture when you tune to an HD channel.
Also, this question is in the wrong forum. This forum is for Comcast boxes with the TiVo software on them, otherwise known as ComcasTiVos. You'll get more help if you ask this question in the Help forum. Just FYI.
Hope this helps,
1+ Pioneer DVR-810H-S
1+ Pioneer DVR-57H
1+ Toshiba RS-TX20
1+ Humax DRT400
1 Humax DRT800
Most with Lifetime and upgraded drives
All Series 2s with Linksys USB200M