VPN client DNS order issues

[Luke]

Hi,
We have a bunch of clients running XP who connect to our 2003 VPN server.
Previously this was working fine but recently, when they connect the DNS
servers dont resolve the private network names.

When doing an IPCONFIG /ALL our DNS servers are listed against the VPN
connection, but when running an NSLOOKUP it keeps querying the DNS server
belonging to the ISP.

I have seen a few articles about mucking about with the registry and such,
but I have worked in IT for a long time and dealt with a lot of different
networks and never hit this issue (DNS always just works when connecting to a
VPN).

Is there anything I could check from the server end, or is this limited to
client configuration?

Thanks in advance for any light you might be able to shine on this problem.