In my experience, I find Media Center the most sensitive area in Windows 7 as far as getting it to work. I have Windows 7 installed on two systems here and Media Center is working on both of them. They are both using ATI Theatre 650 pros for tuners.
I downloaded VISTA drivers
for the tuner and the display adapter, saved them to my desktop, and installed them in compatibility mode for Windows Vista. I ran the driver
installation programs "as administrator" as well. Do you know how to do this? If not, I can explain.
Once I installed those drivers, all of my problems went away. If you haven't tried this, I suggest you do.