I have posted a few items in the past related to the Xbox 360 PC setup package that can be used to configure an Xbox 360 as a Windows Media Center extender. In order to make it easier to find all of the suggestions I have posted, I created an article with links to each of them. You can take a look at the Troubleshooting guide for Xbox 360 PC setup issues to see the previous posts I have written on this topic.
One important thing to note that is listed at the bottom of that article is that the Media Center Extender team at Microsoft asked me to let everyone know that they monitor the newsgroup named Microsoft.Public.Windows.MediaCenter to search for Xbox 360 PC setup issues. If you run into issues installing or configuring an Xbox 360 as a Windows Media Center extender, please search that newsgroup for similar issues, and if none are found, please post a new thread there so that the team will be more likely to see the issues and respond to them in a timely manner.