If you never want that user to access anything other than Media Center, I'd set MC20 as the shell.
I would strongly recommend that you set this for the user account and not for the system, and only if you're really sure that's what you want.
Rather than edit an existing user (though you can) I would recommend creating a new user account with admin privileges (at least temporarily so that you can edit the registry) and while logged in as them, open up regedit and navigate to: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\
Create a new Key called System if it does not exist.
Now add a string called Shell with the value of:
"%SystemRoot%\System32\MC20.exe" /theater
Now when you log in with this user, Media Center will be the only thing that is launched, and it should launch immediately - this is because you have now replaced Explorer with MC20 on that account.
Be warned that doing the above could go quite badly wrong and you do this at your own risk, which is why I recommend that you do it on a separate user account.
You can then configure that user for automatic log-in when the system boots, if that's what you want.
As a way to "rescue" yourself if something does go wrong, it should always be possible to hit CTRL+ALT+DEL, bring up the task manager, and then run explorer.exe
Everything should be fine - I just did this on my own system with a new "MC20" user account as a test before posting it - but you were warned.
