Cannot Login Using MS VPN For Specific User

Hello.

I am trying to login to our Win 2003 server using a VPN connection setup
under "Network Connections" on my XP machine.

It works great, as long as I use a particular user. If I try another user
name and password I get "Windows was unable to connect to the network using
the user name and password you provided. Please re-enter the user name and
password."

Things I know:
- I'm using the correct user name and password because I can logon using
them with "Remote Desktop".
- The user is part of the group "VPN Users"

Any ideas?

Thanks,
Blair