Quick answer.
Can't ben done. Media Center does not support (there might be a hack someplace, not sure) HD though a cable box. So with your cable box, you will only get SD programming. If you want to get all your HD cable channels, you need to get a NEW computer that supports cable cards.
If you want to get some HD programming, You can get locals via an antenna (OTA), or cable "QAM" channels with a box called the HDHomerun.
QAM will work right now, OUT SIDE OF MEDIA CENTER with your HDTV card software, Media center can't use it.
-Dave
MCP, MCSA, MCSE 2003
Windows Vista Connected Exp:Home Theater for Technologists
Windows Vista Connected Exp:Home Theater for Sales professionals
Home theater specialist (10+ years)