After installing CRM 3.0 server and the Outlook client on multple pc’s, running Outlook 2007, we had some users complaining about having to login multiple times in the Outlook Client for CRM. We already made sure the CRM server was in the Intranet Zone by editing the domain policy, and logging into CRM with Internet Explorer worked fine for all users.

The following KB article is how we solved this.

You are prompted multiple times for the user name and for the password when you start Outlook or when you click any of the Microsoft Dynamics CRM folders in Outlook

http://support.microsoft.com/kb/934243

I still think it’s strange that not all clients had these problems, but we also discovered that some of these users were local admin on their machine, so that might have something to do with that.