Outlook/Network Password Issue

I have 2 users who have recently started getting the same problem. They log on to our network just fine, but then, when they try to open Oulook (2000), they are prompted to once again to enter there password. After they enter there password, they get a “credentials conflict” error, and are locked out, and I have to unlock them. I have never set up any such feature, and I can find no documentation on how to disable this. Any help wpuld be greatly appreciated.