Question : Default gateway and NAT

I have two sources of internet
1) SDSL modem connecting to internet ZyXEL Prestige P791R SDSL Router.
2) ADSL connected directly to windows 2000 server which is configured as RRAS server
for routing and PPTP connections.

clients are connected to the internet using ADSL on the server (default gateway is windows 2000 servers ip)

I want to use SDSL line to port forward to a machine on network for using as web server or VNC connection. I configured to the SDSL routers internal interface with a static Ip and configured port forwading.

Everything is ok but it works only when i configure the SDSL line as default gateway.

Clients can browse internet and perform normal functions but i cannot use port forwarding until unless i configure the SDSL line as default gateway.

Point here is i dont want clinets to use SDSL as their defaut gateway. It will be used for dialing in only.(in future) and web hosting.
Whats the problem here???
I understand the way nat works clinets do not need gateway to communicate with routers internal interface.

Answer : Default gateway and NAT

Port forwarding is a "subset" of NAT (NAT can be considered port forwarding for all ports).

The definition of gateway is: the address that must be accessed when the requested address is not in the local network. A computer can have more than one default gateway, but it will use the one with the small metric first and the other one wnhe first fail.

Add the second gateway to nic's tcp/ip configuration.