Help on VPN over NAT !!!!

I have problems in using VPN over the NAT,

Here is the scenario:
I have DSL cable connection and the router is Aztech Turbo 900, I configured a Windows 2000 VPN server on one of my machine on LAN having two LAN cards.
I configured my router for NAT and forwarded TCP port 1723 to the VPN server's IP address.
Now if I try from any machine on the LAN to connect to that VPN just to test, it works and the client gets connected. But if I try from outside, it goes till verifying the "usernameand password" after that I get a "Error 721: The remote computer is not responding"

How can I resolve this? Can I use VPN over NAT?

Any help would b appreciated. If I am following any wrong process, a detailed link will be appreciated.

Thanks in advance