Basically, on the client side you would have to clear the proxy setting in your users' web browsers, give their computers DNS server addresses for Internet-name-to-IP-address resolution. Also, you have to set their default gateways to point to either the PIX inside interface, if it sits on the same subnet as the users and there are no other internal subnets; or to a default gateway on their subnet which uses the PIX inside interface as its "gateway of last resort".
On the PIX, you would need to set up NAT/PAT, and either outbound (old) or access-list (new) commands to permit the users on the inside to go to the outside (Internet) per company policy.
For what its worth, you could set up the ISA server to act as a caching proxy server with only one NIC, or reconfigure it with two NICs to have its outside (or "north") NIC sitting on a VLAN that also contains the PIX Firewall's inside interface, effectively putting the ISA behind the PIX. Then you could use the ISA or go direct, depending on your application from the user's side. In either case, this would get you the extra IP address that's on the ISA server now.
Would need to know more about your network to supply any detailed assistance, though.
Hope this helps.
