For premium cable content, you need a CableCard tuner. The only one out right now is the Ceton one with 4 tuners. It is expensive though. Silicondust is coming out with a less expensive CableCard tuner (HDHomerun Prime) but who knows when it will be out.
If you do get a CableCard tuner, you need to rent a CableCard from comcast. You won't get on demand programming though.