Unable to request remote assistance

Since I reinstalled SP2 from the full installation file I have no
problem accepting remote assistance calls from others. Tonight,
however, I wanted to show my daughter something on my machine and tried
sending her a remote assistance request to that end. She accepted the
call and the remote-assistance loader got as far as the "connecting to"
message but I never saw anything at my end. I am behind a NAT router
by port 3389 is forwarded.

Other things of potential interest. My main machine is part of a
domain and I have a VPN (PPTP) connection running on it. Remote
assistance is through MSN Messneger 7.5.

Any ideas what is going wrong here?

PS. Running XP Pro/SP2 and all critical updates. Port 3389 is
forwarded on the router and both the XP firewall and my anti-virus
firewall are definitely disabled.

Tried sending a remote assistance request from a laptop on the same
home LAN. It worked! Also the laptop was connected to my VPN so that
has nothing to do with the problem. Whatever it is, it is specific to
my main home-office machine and it has survived a complete
reinstallation of SP2 from the full installatiion file.

Does anyone have ANY ideas????