Now you are on turf I am not sure about. Probably could figure it out in an hour at your home... but no dice other than something not being setup correctly, or something being fried.
I believe, even with 'basic' cable, you should still be able to receive the HD channels from Comcast. They are on the line and never scrambled... I think. I have digital cable, a DCR plasma, and no CableCard. Cox delivers my NBC, ABC, FOX, and CBS crystal clear to it. I would try that out.
Also, many TVs have two coaxial connections on the back. One is for a cable connection, the other is for an antenna. Not sure with your TV though.