• "An authentication error occurred while communicating with the web server."
• "Debugging failed because integrated Windows authentication is not enabled."
• "Authentication error occurred while communicating with the web server.  Try disabling 'Digest authentication'"

The above errors occur because of the way that VS 2005 looks up the Process ID of the IIS7 worker process that ASP.NET is running within.  Specifically, when you use F5 to "auto attach" the debugger with Visual Studio it sends an HTTP request to ASP.NET using Windows Authentication to retrieve the worker process details.  This works fine if you have Windows Authentication enabled on your web-server, and are using Windows Authentication as the primary authentication method for your web application.  It runs into problems, though, in a couple of circumstances:

1. If you have forms-auth enabled in ASP.NET and are running in "integrated mode" on the IIS7 web-server.  This ends up blocking the process handler.
2. If you don't have the windows authentication module installed on your web-server (it is now an optional component).
3. If you are running on Windows Vista Home (which doesn't support the windows authentication module).