We recently ran into a situation with a user who was continually being prompted for a username and password when starting up Explorer. We worked out the mysterious prompt issue, and thought we'd share its resolution in case others are experiencing something similar.

Behind the scenes, ArcGIS Explorer is connecting to ESRI's ArcGIS Online services, and uses a transparent username and password in that connection. But in this particular case, Explorer would display the following message on startup:

What we discovered is that this was caused by a change in the default User Authentication settings for Internet Explorer. If you are experiencing this problem, here's how to correct it.

First, startup Internet Explorer and open the Internet Options dialog.

Choose the Security tab, click the Internet zone, and scroll down to the section on User Authentication. To correct the problem we changed the setting from Anonymous logon to the default setting of Automatic logon only in intranet zone.