Ok. If i have this right. Media Center displays your webcam and not TV when ever you tell it to watch live tv.
The only thing I can see is maybe windows took your web cam as the only device with picture when media center was configured. So lets try to change that.
First close media center if it is open.
Disable or unplug your webcam
Open media center and navigate to Task -> Settings -> TV -> TV Signal -> Set Up TV Signal.
Follow the instrucations and make sure you have cable or an antenna connected to the coixial connector on the card.
If you are running with a Cable/Sat box, you will want to use the composite input(yellow) with the audio inputs(Red/White). For that would also require a media center remote and IR blaster(they can be bought together, but not all have the ir blaster, you need that if you are running it off a cable/sat box).
With any luck as you work your way through the setup, the TV Box should display an image and be configured. After this is done, Test your Live TV and fm radio.
If all is good, close media center and re-enable or plug in your webcam. From this point on, Live tv should work. If media center tells you of another tuner at this time, just do not configure it.