I’ll assume you’re talkinga about domain users here, as in Active Directory. When I have a user who can’t open Outlook, especially, I know that user really is not logged onto the domain. If this always involves the same computers, you might try taking the machine out of the domain and putting it back in, or resetting the machine account. If you’re really sure it’s the users, there’s some problem with the user’s account, or for some reason they’re not authenticating to the domain, but rather to the local machine. I would delete the user account and re-establish it.