PPTP uses account based authentication. I don't believe there is a way (available now) for SME to authenticate through Active Directory on the W2K Server.
Winbind (part of current release of Samba) does allow for remote authentication of users to a NT primary domain controller, but I don't think this works for Active Directory.
As much as I hate to point you this way...Since you have the W2K Server, why would you not want to use the built in Routing and Remote Access Server (RRAS) which can handle the PPTP, or L2TP VPN for you? Then you have standard access to dial-in rights for the users and groups you want to allow, as well as enabling appropriate dial-in hours. What would SME be adding to your network in this case?