How do you do apps share with a Vista machine using Office Communicator?

We use Communicator 2005 and LCS for IM here. XP is our standard platform.
Messenger 5.1 delivers the applications sharing component of Communicator.
It does not seem to be a component of Communicator itself.

This all works fine until we try to do an apps share in Communicator with a
Vista machine. Messenger 5.1 will run run under Vista, so we just get an
error saying either ""%username% does not have data sharing session
installed(or it is disabled) and is not able to accept your invitation." on
the XP machine or "A data sharing session could not be established with
%username%. Please try again. If the problem persists, please contact your
system administrator" on the Vista machine.