MODX 2.2.2 on IIS 7.5 - Authentication Required (Manager login)

On a recent install of MODX 2.2.2 attempting to login to the Manager threw a Windows Authentication dialogue box.  Selecting Cancel allowed access to the login screen on internal browsers (FF, Chrome and Opera) but could not be bypassed at the client's location.

Disabling Windows Authentication in IIS for the client's website prevents the Authentication box from being shown.

Interestingly Firebug showed that all the required files associated with the login page were sent to the browser even though the Authentication dialogue appeared.