I've tried all the labs for the course 2826, and the ISA 2004 server in each of them has a problem, in that the WebProxy service is not able to run. Try completing the very first lab, and you'll see. The only thing I could find left to try was reinstalling the product, but of course the product CD is unavailable.
This error persists across all the labs, leaving them all 'broken'
Easiest way to check: On the ISA Server, run netstat -aon | find ":80" at a command prompt - there should be an entry for TCP Int.Ern.Al.IP:8080 0.0.0.0:0 LISTENING nnn