Not Authorized to View This Page [CFN #0004]

Whether it is a simple one-page of HTML, or ASP.NET 4.0, my IIS 7.5 displays this error message.  

        Not Authorized to View This Page [CFN #0004]

I've found Microsoft's value-added HTTP Codes 

http://support.microsoft.com/kb/943891

and Microsoft's System32 Error Codes 

http://msdn.microsoft.com/en-us/library/ms681381.aspx

And seen wfetch.exe reporting the HTTP 200 error (success, your HTTP has displayed).

I appreciate this is going to be a really "obvious" issue, probably to do with authorization and authentication.  This error message displays, whether I'm in IIS Manager and click on the Browse Web Site, or use IE or Chrome, or another coimputer on my home network, or get friends to try from the WWW.

Thanks