I have just been looking at this guide written by Ryan Tollefson over at the Media Center Guides website, which enables you to make a VMC PC more appliance like by only allowing users access to the Vista Media Center UI and nothing else within the operating system.
I can confirm that this does indeed work having just tried it myself today. There is one known limitation however:
"The only known issue is that you'll not be able to change or mute volume because it depends on explorer.exe"
In my tests mute did work but changing the volume did not.
But if you are using an AV amp to control the volume level of your VMC system then this might not be so much of a problem. You can also see the thread here about this guide on The Green Button.