Mapping a drive Dialing a threw VPN

I am dialing from a win2k pro SP3. When I dial my ISP, then use a vpn connection to dial into an nt 4.0 server. I try mapping a drive and It never finds the share. I get an error message “The remote computer is not available” I am able to ping that server and also bring up my email. Is there a way to configure what port is uses when mapping a drive. Currently is using port 445 and I would like to change it to port 139??? Help