Hi brianshinn,
- Do you receive any error codes or messages?
You can refer the following links and check if it helps to setup Xbox:
Set up Windows Media Center on Xbox 360 (Windows Vista)
How to configure network settings on an Xbox 360 console
http://support.microsoft.com/kb/967572
You can also refer the following article:
Set up a Windows Media Center Extender device
Troubleshoot problems with a Windows Media Center Extender device
Stream media using Zune or Windows Media Player
Hope this information is helpful.