From: Rhoadesville, VA
It seems to me that if your ADSL router is already using 10.x.x.x addresses, then NAT is already being performed by the ADSL router.
Two more assumptions: I would assume you want all traffic to go through the ISA box and, in this case, I would also assume that the internal IP of the router is now going to be the gateway IP for ISA's external NIC.
The external NIC on the firewall has to point to the Router. Therefore, it's on the 10.x.x.x network.
The internal NIC on the firewall is pointing to the local network. I would recommend reconfiguring this network to use a different range of IP addresses. In this case, set the internal NIC on ISA to 192.168.0.1 and configure all of the internal clients with 192.168.0.x addresses.
Make sure the internal network clients use 192.168.0.1 as thier default gateway.
I hope that helps... If you have any additional questions, please feel free to ask. -Paul