VERY odd routing behavior when attempting VPN connections over Wifi

I am having a really weird issue occurring for some of our users who are
doing VPN into our corporate network via WiFi.

We are using Nortel's Contivity VPN solution between our client machines
and our corporate network. Everthing has been working fine until a
recent Exchange 2003 migration. We moved users mailboxes from an old
Exchange 2000 server to a new Exchange 2003 Server. When this move
happened, a number of our home users suddenly couldn't ping or route to
the new Exchange 2003 Server (which resides on the same subnet as the
old Exchange 2000 Server.

When I have the user try to do a ping or a traceroute to the IP address
of the new Exchange 2003 Server, over VPN via WiFi, their host system
resolves to the MX record of our public DNS domain zone.

The common factor that I've been able to determine is that the people
having the problem are running on Windows 2000 Pro or XP machines that
they did in place upgrades from Windows 2000 Pro. Any users using a
clean install of Windows XP, are working flawlessly.

Any ideas?