Vista Premium Media Center can't pick signal. HELP!

Hi all,
I'm a newbie when it comes to the AV world. So please bear with me.
I recently bought OnAir HDTV GT tuner for my PC.
Using the software provided, I can pick up channels through direct cable
connection (digital comcast) and through an antenna (provided by OnAir).

But when using Vista Media Center, I can't pick any signal nor channels when
using direct cable connection. However antenna works just fine.

The error message that I get is: "A TV signal was not detected"

I have -however- done these following:
1. Double check if the coax cable is securely tightened.
2. Installing the latest driver from OnAir.


Any help will be greatly appreciated.

Thanks