Media Center remote/guide won't change Cable channel

Hello!

As things stand, I can watch TV on the snazzy Media Center, but I cannot
change the channels with the MC channel guide, or with the MC remote. Despite
wading through the set-up a bajillion times, in every possible permutation...
nope. Can't change the channels other than using the Comcast STB remote to
change the channels on the STB... that is, I use the Comcast remote and STB's
guide, and bypass the MC channel guide/remote entirely... which is a drag,
and not what I want to do, clearly. So the channel I'm watching, and the
channel the MC guide says I'm watching have nothing to do with each other. I
can change the MC guide to any channel, but it only displays the channel the
STB is actually set for.

It doesn't seem to be an IR blaster issue 'cause even once "programmed" the
MC remote won't operate the STB at point-blank range.

The STB is connected to my HP Pavillion via coax... should I switch to
component?

Ummm, help?