Six_storm for help with windows media center you may want to visit www.thegreenbutton.com
(click on community to get to the forums).
Otherwise the problems your running into are somewhat like this:
-All cable and Satelite companies encrypt their HDTV signals.
-The klunky way to get a HDTV signal is by buying a tuner and having it control your cablebox (which is currently the only viable method).
-The non-clunky (pain in the ass) way to do it properly is with a capture card that will fit a "cable card" decoder.
However, the cable companies in the USA (because this is specific to you) while being forced to support cablecard, really don't. The only way to get a cablecard system is to buy one retail and have your cable company send someone out to put the card into your pc (yes, it is that stupid). Plus, it really doesn't work that well.
A good read on the subject: http://arstechnica.com/articles/cult...et-top-box.ars
Sorry that I don't offer any solutions for you.