Vista - Unable to connect to VPN using Vista and Nortel Contivity

I got a new Dell Dimension 9200 running Vista Ultimate about a week ago and
gave been trying to connect to my work VPN ever since using the Nortel
Contivity VPN. I found the version of Contivity that loads on Vista and I've
got it up and running. However the connection comes up for about 15 seconds
or less before dropping out with a message that the secure connection has
been lost. My trawling on the net suggests this is as a result of an
inability to send and receive two-way traffic on UDP 500. Running the exact
same Nortel client through the exact same router with my Dell XP machine
works just fine. Must be something with the Vista machine! I've checked the
firewalls, I've connected to the internet without the router and I just
can't make it work. It's driving me NUTS. Can anyone tell me how to make
this work?

PS This thread shows that at least some people have got it working
http://www.tek-tips.com/viewthread.c...1280316&page=1