ISA 2004 by default is very secure. What I mean by secure is that it locks everything including your internet acccess after a clean install. You need to go to the "firewall policy" to enable them. You will also need to separately enable the dns services, DHCP traffic, you name them, if appropriate.......... In other words, all services or traffic going through the ISA box is blocked.
Go to this website:
Microsoft ISA Server Firewall Resource Site: Articles & Tutorials Provides alot of indepth discussion on isa 2004 by the ISA expert Tom Schinder
http://www.isaserver.org/tutorials/Installing-Configuring-ISA-Server-2004-Enterprise-Edition-Part1.html
The"Related Links" on the page are also useful references.
Here's also another link:
http://www.techtutorials.info/2003isa.html
Note that some sub-links from this link requires you to buy thier books.