I wanted to test Windows 7, a website with Windows Authentication.

Normally it is sufficient, Windows Authentication in "Programs and Features" / "Windows Features" to activate

and then in the IIS Manager to enable:

When you call the page, however, to my astonishment, I received a popup to enter the user data:

After I entered the credentials three times, I received the following error:

The reason for this is that I've given the site its own host header.

Produces the result that the loopback check, which was introduced with Windows XP SP2 (and apparently also in Windows 7 is used), fails, and the above error message with the Error Code 0xc000006d.

The behavior and the remedy is in
KB896861 described.