Microsoft have launched a dedicated Media Center forum on the Microsoft Answers site. You may wonder why Microsoft have created the forum when they have The Green Button but I think Microsoft’s reason is that TGB tends to be focused towards enthusiasts where as the Microsoft Answers site is for end users.
There are no posts in there yet but I expect it to start filling up with codec questions soon
http://social.answers.microsoft.com/Forums/en-US/mediacenter/threads