application sharing problems.

I am having problems with application sharing using
windows messenger 5.0 with another friend with 5.0 as
well. I have win xp pro connecting through a linksys
router wrt54g and my friend has win xp home using a
linksys router W11S4PC11. We can share messages, ftp, and
remote assistance both ways. But, when we try using
application sharing it does not work at all. I have
upgraded my routers firmware to the latest and it supports
uPnP, but my friend's doesn't have an upgrade. We tried
forwarding a port (don't remember the number) but it did
not work. We tried application sharing when we were
connected through a university's lan and application
sharing worked, although I could only do so and not him.
One time I was able to bring up application sharing using
the internet, but I was not able to click on any program
in order to share it.

I would appreciate if someone can help me out on this
issue, since I don't have any prior experience with
messaging programs.

Thanks,
Martin.