Users are asked for Outlook (or MS Communicator) login every morni

We're running Outlook 2007, Communicator 2007, and Exchange 2007.

Each morning our XP users are prompted for their domain user credentials to
log into Outlook/MS Communicator. We believe it's the integration between
Communicator & Outlook, but we're not 100% sure.

If a user shuts down both Outlook & MOC at night, then they won't see the
login prompt in the morning, even after restarting the apps. If they only
shut down either only Outlook or only MOC, then the login prompt will appear.
And if both apps are left running overnight, the login will appear in the
morning.

This only started occurring only within the last few months, and we've had
these 2 programs running for a while before that. We did do some
administrative work on the Exchange side since then.
Is it possible this issue has to do with Autodiscovery or Public Folders?

We'll be glad to provide further info if needed. Thanks in advance.