Could you please provide more details on this?
What Media Center software is being used? Is it Windows* media center or something else? When does the issue happen? Does it happen when you just launch the application or when you try to play something?
What driver versions are being used? did it work with a previous driver release? If so, please specify the version that works and the one that does not.
Does the system recover to 32 bit when you close this application?