Both the web proxy and firewall services fail to start after installation and will not restart. Event viewer gives the error code as #7031; "The Microsoft Web Proxy service terminated unexpectedly."
When I've run into this problem, I've had to remove ISA Server using the rmisa.exe program on the ISA Server CD. Run that program and then restart the computer and try again.
Its seems to happen more often when the machine RAM or disk space starved. Another trick to try out is to disable or uninstall IIS from that machine.
If you still get the same problem, try re-installing the OS and ISA Server