Something or someone could be attempting to logon automatically and repeatedly via stored credentials with an incorrect password. To determine if this is the case or if in general there’s failed logons check the audit logs on the server (Control Panel->Administrative Tools->Eventviewer->Security) and look for logon failure events.
Given you mentioned Outlook, it could be that the user has setup outlook to logon using the username/password rather than Windows integrated authentication. If this is the case you should set the user up to authenticate to Exchange via Windows integrated authentication thus avoiding the problem and increasing security.
Hope that helps.