I recently installed .net 4 on an IIS 7.5 box and after registering it found that IE wouldn't login to any v4 sites using Windows (Integrated) Authentication. It would prompt for a password until eventually returning a 401.1. Firefox worked and so did IE from the IIS box itself.
The solution was to move the "NTLM" Authentication Provider to the top of the provider list (above "Negotiate"). Goto your site->Authentication, in actions click "Providers..." and move NTLM to the top.
Anyone know why?
