So you are trying to access this WinServer2003 Terminal Server from a remote location(Outside your local network)?
The question is that are other users trying to access Terminal Server within the Private Network via VPN having the same problem?
You can enable the Terminal Server services on WinServer2003.
Follow these steps to enable Terminal services on WinServer 2003:
1) Start—->Settings—>Control Panel—-> Add/Remove Program—->Windows Component—>Terminal Server.
After enabling Terminal Services on WinServer 2003, check and see if the problem is resolved.
I hope this helps