I’ve been playing around with the Media Center SDK over the last couple of days and ran into a small issue when using C# Express Edition. When you create a new Media Center add-in you have to get a Public Key for your application. This needs to get added to your register.xml file so your application gets properly registered within Media Center. The readme file in the Media Center Application Template for Visuall C# explains how to do this. The one catch is that they assume you have the full Visual Studio 2008 installed.
Long story short the readme file says that the target path for your sn.exe is C:\Program Files\Microsoft Visual Studio 8\SDK\v2.0\Bin\sn.exe, when it should be C:\Program Files\Microsoft SDKs\Windows\v6.0A\bin\sn.exe if you are using Express Edition.
Took some digging to find that.