VPN

I am trying to connect to a Windows NT 4.0 server using
VPN. I have configured the PPTP protol for the server and
the 98 machine. both are behind firewalls. I have opened
the ports required. On one, I set my computer as the DMZ,
and on the other I opened 1723 and 47. When I try to
connect from XP i get error 721, and on 98 I get error 650

What's wrong?