Microsoft Knowledge Base Article 816320 contains the following summary:
Internet Security and Acceleration (ISA) Server supports
several client types,
including Web proxy client computers and firewall client
You can also configure a CERN-compliant browser to be a Web proxy
Web proxy client browsers send Hypertext Transfer Protocol (HTTP)
requests directly to the Web Proxy service on the ISA Server computer.
with the Firewall client installed forward Winsock
requests directly to the
Firewall service on the ISA Server computer.
You can manually configure both the Web proxy client and the firewall client with the Internet Protocol (IP) address or the computer name of the ISA Server computer. However, if you manually configure the address of the ISA Server computer on mobile computers, the configuration may not be correct for all sites that are used by the mobile user. Autoconfiguration is a feature that makes it possible for the Web proxy and firewall client to discover the address of the ISA Server computer. This feature is also known as Autodiscovery. Autodiscovery makes it possible for mobile users to connect to the ISA Server computer that is appropriate for the site they are in, without manually reconfiguring their client settings.
This step-by-step article describes how to configure the Windows Server 2003-based server and the clients to use Autodiscovery.