VPN and map drive

I am trying to solve a problem with VPN and mapping a drive. The server is Windows 2000 server. I have tried with multiple clients from multiple sites and get similar results and different user accounts, win 2000 and XP clients.
I can successfully authenticate and get registered on the network. The problem starts when I try to map a drive with net use, it asks for a password and then reports a bad user name or password.

Please help, very desperate.
David