win2kpro as a router

I have win2kpro pc that has 2 nics. The first nic (nic A) connects to my regular network and has its gateway defined. Through this nic I can access the internet and the rest of my network from this win2kpro PC. The other nic (nic B) is connected to a hub with its own separate IP network using a different IP address scheme and is the default gatway for all devices in this separate network. I have enabled IP forwarding through the registry. From a device in this separate network, I can ping the first nic (nic A) which connects to the regular network but I can not ping any other devices in the regular network. What can I do to be able to ping any other device in the subnet that nic A is connected to from the separate network that nic b is connected to. Thanks