I can PPTP VPN into our SME gateway, and have the client authenticated with the NT4 PDC on the LAN behind it. I'm assuming you already have the NT box providing DHCP and stuff to the LAN...
First the SME will be handing out the VPN IPs, not the NT box. Configure the SME to use whatever range you want (probably the same as the NT box is giving out, or a subset (x.x.x.240-x.x.x.250) of that). Then run the configuration again, and disable DHCP. It will remember that range, and use it (the top end of it actually) for PPTP connections even though regular DHCP is off.
Specify the WINS server to the SME using the instructions at
http://forums.contribs.org/index.php?topic=4529.msg15729#msg15729 (I think this gets passed on to the client, but I'm not sure). It can only help...
Finally, make sure you choose the option to logon to or specify the domain in the Windows VPN connection. On XP, it adds a Domain field to the prompt screen, and 9x pops up a logon box like when I boot on the LAN...