You can add a text message that appears in the instant messaging pane of the Conversationwindow every time an instant messaging session starts.

To do this, use the IMWarning policy key under HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Communicator. This policy can also be set in HKEY_CURRENT_USER\SOFTWARE\Policies\Microsoft\Communicator, but settings in HKEY_LOCAL_MACHINEtake precedence.