For Windows Media Center (or Centre) to control your cable box, you need to
have the Microsoft Remote - or perhaps third-party remote.  

The Microsoft remote I use came with two IR emitters that plug into the
remote sensor.  You stick one of those emitters in front of the IR sensor on
your cable or digital box.  Next, you configure Media Center to control the
digital box by playing with your remotes and settings until Media Center
sends the proper codes to your digital box via the IR sensor.  Over all, the
process is a lot of work and not very reliable.

To control your digital box without the IR emitters, the manufacturer of
your box would have to provide a Media Center add-in and the box would have
to connect to your PC through, perhaps, a USB port.  I am not aware of any
boxes that do that but then I am not a box expert.  I know that Dish Network
does not have any such box and have stated that they do not have any plans of
offering a box that interfaces directly to a PC or to Media Center.

It turns out that, for many people, Media Center only works well for
television use if you have a tuner and over-the-air broadcast reception.  
That is one of the reasons I abandoned the idea of using Media Center almost
as quickly as I rushed to try to use it.  The idea and the hype is terrific.  
The reality is not as great.

Dale