Aaron - this is an issue which is called out in the Known Issues section of
the download page.
Here's a KB article describing the problem:
http://support.microsoft.com/kb/892374 Windows Media Center Extender cannot
establish a remote connection after you install Windows AntiSpyware (Beta)
on a computer that is running Windows Media Center Edition 2005
I believe the workaround is to uninstall Microsoft Antispyware.