I can tell you this much. DVB-C is a european standard that should be corss platform, so in theory you could get:
DVB-C card (although it must have correct BDA drivers to work with media center) also have a slot for a CAM
- A Cam of the correct encryption type, I think it's nagravision.
- A valid NTL subscriptions with NTL Nagravision card.
Now the NTL Nagravision card locks to the Nagravision serial number in the in-built CAM in the NTL set top box, so it wouldnt' work unless you could some how change the Nagravaion number in your sperate CAM to the same one of your set, top box.
At the moment they don't make DVB-C cards that have BDA drivers, so no chance.
However I'll bet within the next couple of years, NTL are supprting this as a feature, and your ringing up NTL to get a new viewing card for your media center PC.
I have Media center connected to my NTL set-top-box, it changes the channel fine and captures via my hauppauge pvr-250. I've got a widescreen projector connected and it's amazing in use. For now I'd just use your set-top-box, the image quality is still excellent despite the Digital-Analogue-Digital conversion.
see
www.thegreenbutton.com or
www.xpmce.com for more infoormation about media center working with NTL.