X,
You don't need to enable Routing and Remote Access on your server.
You just need to put a default gateway the External NIC (do not change the metric) and static routes to all your private network through your internal NIC.
To configure the static routes first you need to find the Internal NIC instance, it can be done with the command "route print", it will show you something like this:
Interface List
0x1 ........................... MS TCP Loopback interface
0x20 ...00 13 02 d1 1b b4 ...... External NIC name
0x452 ...00 16 d4 41 66 76 ...... Internal NIC name
Then add the route with the command (assuming your internal router is 10.0.0.1):
route ADD 10.0.0.0 MASK 255.0.0.0 10.0.0.1 METRIC 3 IF 0x452 -p
The -p is to preserve the route after a reboot.
Hope it is clear.
