Ok so here is what you can do (I dont have your network diagram so please bare with me).
Network 1 (N1). We will say is network 10.0.0.0 255.255.255.0
Network 2 (N2). We will say is network 192.168.0.0 255.255.255.0
ISA Machine. (ISA). We will say it is on its own network of 172.16.0.0 255.255.255.0
Take your lawyer 3 switch.
On interface 1, give it the ip address
10.0.0.254 (your gateway address or default route address for that subnet). Plug N1 into Interface 1.
On interface 2, give it the ip address 192.168.0.254 (your gateway address or default router address for that subnet). Plug N2 into Interface 2.
On the last interface on the switch give it an IP address 172.16.0.254 (your gateway address or default router address for that subnet) Plug ISA into that interface. IP the server is 172.16.0.1
N1 all machines on N1 have a default route of 10.0.0.254
N2 all machines on N2 have a default route of 192.168.0.254
ISA, Now we need to add a couple of routes to this machine. You need to add
All to point to 172.16.0.254.
You will also need a default route pointing to the external interface (if directly connected to the net or to a router).
The ISA machine can nat for all the 10, and 192.168 address if you want. OR you can put routes on the router so you are only NATing once.
The switch will need a default route pointing to the ISA IP (172.16.0.1)