VPN W2k Loggin across the internet

I got a client(XP pro) to Lan(w2k domain) VPN connection working with a client(ssh) program and a VPN router(multitech). I can browse the shares(on a specific computer) using an IP address and domain credentials. My question is how can I get the remote client to join the domain and have users actually login to the domain?? Multitech says this isn’t possible….I don’t believe them!
Any Ideas??