I'm having the same problem. It started yesterday. I tried turning DEP off for the Media Center App but Vista wouldn't allow it. Just wanted to let you know.
I also went to the Netflix website and deactivated all PC's that I had attached to my netflix account and then signed back into netflix from my media center. That didn't work either. That's when I noticed it's not even getting to the part where it checks for activation. Media center is not able to make it that far.
Netflix reports that everything is working correctly from their end. So from my point of view either Windows did an update that broke it or Comcast is throwing a monkey wrench into it.
On support.microsoft.com there is a diagnostic tool that may be helpful for you. As soon as the page loads you should see a link to the tool under a link titled "Automatic fix for Windows Media Player video and other media and library issues"
Yes, I am on Comcast. Strangely, 2 days later I am now able to view Netflix movies again. I am also back to my original issue of having the movies having a border around them. When I search movies on Netflix, I am vieiwng everything in full screen, but as soon as I start movie playback, I have the border around the picture.