I'm not the brand manager for Windows Media Center, but I did what Jackie Huba said to do anyway and found this: http://www.youtube.com/watch?v=rQKI7YvtLhQ&search=Media%20Center at the top of the search results for 'Media Center'. Pretty cool romp through our user experience.
Update: These are still kinda lame (especially compared to iPod + iTunes commercials) and don't actually speak to what Windows Media Center is all about, but better than the general 'Start Something' campaign.
http://www.youtube.com/watch?v=8NDSKAa4WMI&search=Media%20Center
http://www.youtube.com/watch?v=ve1OEey8ryA&search=Media%20Center
I'm beginning to see why awareness of Windows Media Center is pretty low, if these commercials are 'the best we could do'.