Application Sharing failure (one way only)

I am using the Exchange version of Messenger 4.7 for
Windows XP to accomplish an application sharing session
between a SoHo user in California and a conference room
workstation in Massachusetts.

Both computers are running Windows XP and the same
version of Windows messenger. When an application sharing
session is created from the SoHo laptop in California the
session can be created successfully and the application
can be shared with the workstation in Massachusetts.

The problem occurs when the application sharing session
is initiated on any one of our conference room PCs in
Massachusetts. The application sharing session begins,
the invitation is sent and replied to successfully but
when the window is displayed to pick applications to
share any available application is grayed out and nothing
can be picked (so nothing can be shared).

A little background, if important. We are running a
Windows 2000 small business server with service pack 3.
At our end we are connected to the Internet via a T1
connection and a Cisco 1720 firewall router.

In California the laptop is connected via a Microsoft MN-
500 wireless router (the problem occurs whether or not
the user is connected wirelessly or hardwired through the
5 port router included in this device. The California
user is connected to a cable modem providing broadband
bandwidth.

This user creates a VPN (PPTP) tunnel into the Exchange
server in order to utilize the Exchange messenger. The
instant messaging portion of the connection works
properly. I can IM her and she can reply. It is only
the advanced features (application sharing, whiteboard
and request assistant) that are giving us problems and
only when they are initiated from our end.

What we have been doing, is having her initiate the
application sharing session and then she relinquishes
control of the session. This works fine but it's getting
a little bit tiresome to always have her initiate the
session. There are many instances when it is more
convenient for us to initiate the session but this is not
possible at this time.

Any ideas as to what could be causing this problem.