Need help reconfiguring Win2003Server NAT router

[VicRic]

Hi. I have currently a Windows 2003 Server named MERCURY working as a NAT router - here's a diagram that explains better the network setting.

Right now MERCURY is working as it is supposed to:
1) JANUS can Ping any other machine on the network and has access to the Internet;
2) other machines on the network, except MERCURY of course, cannot Ping JANUS (v.g. 192.168.3.10);
3) other machines on the network cannot Ping MERCURY's address 192.168.3.1.

I need your help to change points 2 and 3; I'd like other machines on the network to be able to Ping 192.168.3.1 and 192.168.3.10.

The current routing settings are best explained in the following pictures.

Any ideas on what I have to change in the routing setup ?

Thnx