Try this...
"You just need to remove the Media Center Extender certificate first.
To remove the certificate, follow these steps:
1. Click Start , type mmc in the Start Search box, and then press ENTER.
2. On the File menu, click Add/Remove Snap-in.
3. Click Certificates, and then click Add.
4. Select Computer account, and then click Next.
5. Select Local computer, and then click Finish.
6. Click OK.
7. In the Navigation Pane under Console Root, expand Certificates, expand Personal, and then click Certificates.
8. Right-click Microsoft Windows Media Center Extender Host, and then click Delete.
After you remove the certificate, uninstall the Extender, and then reinstall the Extender. Follow these steps:
1. Start Windows Media Center.
2. Click Tasks, and then click Settings.
3. Click Extender, and then select the Xbox 360 console.
4. Click Uninstall.
5. After the uninstall process is complete, click Extender again, and then click Configure.
6. When you are prompted for the setup key, follow these steps: a. On the Xbox 360 Media Remote, press the Y button.
b. Press the Windows Media Center Start button.
c. Note the eight-digit setup key.
7. Return to the computer, and then enter the setup key.
8. Complete the Extender setup.
cheers
Gregools"
I found it on a other site...