Try turning off the OneCare firewall. If the problem goes away, you know it is the firewall - then open OneCare, change settings, firewall, advanced, ports and protocols - at the bottom of the list, select the media extender and xbox option and make sure they are checked.
If it doesn't work with the firewall turned off, seek help in the media center Microsoft Public newsgroups here: http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx
-steve