Pertaining to your case, here is some KB Article listings that might be missing from your system.
We need to have these critical updates for the extender to work.

1. KB958653 - February 2009 Cumulative Update for Media Center TV Pack for Windows Vista - Installing this update will to enable new types of Windows Media Center Extenders, such as digital televisions and networked DVD players, to connect to the Windows Media Center PC.
   
2. KB960544 - February 2009 Cumulative Update for Media Center for Windows Vista
   
3. KB909163 - While attempting to connect your Xbox 360 console to a computer, you receive the error message: "Connection Error"
   
4. KB912503 - While attempting to set up your Xbox 360 console as a Media Center Extender, you receive the error message: "Xbox 360 Setup is finished. The correct ports cannot be opened" (this is also applicable for Media Extenders)
   
5. KB911123 - While attempting to connect your Xbox 360 console as a Media Center Extender to a computer, you receive the error message: "Session Terminated" (this is also applicable for Media Extenders)
   
6. KB911739 (Microsoft Removed) - While attempting to connect your Xbox 360 console to a Windows Media Center-based computer, you receive the error message: "Invalid Connection Data"
   
7. KB959485 - A recorded TV show plays back only audio after you install update 955519 for Windows Media Center on a Windows Vista-based computer that is configured to use digital cable card tuners
   
8. KB955519 - October 2008 Cumulative Update for Media Center for Windows Vista
   
9. KB950126 - June 2008 Cumulative Update for Media Center for Windows Vista
   
10. KB954366 - The August 2008 Windows Vista and Windows Server 2008 Application Compatibility Update is available
    
11. Advanced firewall exceptions and firewall bypass to Media Center Extenders.
    http://www.microsoft.com/windowsxp/mediacenter/extender/setup/firewall/config.mspx
    
12. The required ports needed for XBOX 360 / XBOX Live visit: http://support.microsoft.com/kb/908874
    
13. Update Rollup 1 for Microsoft Media Center Extender for Windows XP Media Center is available from the Microsoft Download Center web site. You can download the update that matches the vendor of your Microsoft Media Center Extender for Windows XP Media Center.
    • Hewlett Packard
      http://go.microsoft.com/fwlink/?linkid=39612&clcid=0x409
    • Linksys
      http://go.microsoft.com/fwlink/?linkid=39613&clcid=0x409
    • Xbox
      Information about how to obtain Update Rollup 1 for Microsoft Media Center Extender for Xbox is available at the following web site:
      http://www.microsoft.com/windowsxp/mediacenter/extender/owner/support.mspx

I have the following updates installed and finally my extender works. For many of us who don't have automatic up dates on for several reasons. Whether you have a web server or you just DO NOT want to break your machine with all the updates. I opened the correct ports and installed the updates below.

I am running an x64 bit Vista Ultimate OS with the following list of updates that fixed my extender:

1. KB950124
2. KB959485
3. KB958653
4. KB957209
5. KB960544
6. KB938126 (x86) or (x64)
7. KB955519
8. KB947172

Required Updates Linksys said to install which I DID NOT:

1. KB950126
2. KB954366

http://www.linksysbycisco.com