Let me see if I understand your question correctly. You changed the HTTP port for JBoss AS from 8080 to 80, and this ran fine on XP, but when you try the same thing on Server 2008 it does not work. Is my understanding correct?
Did you use TCPView or netstat to see if port 80 is opened by the java.exe process?
I assume that you ran JBoss AS with the -b 0.0.0.0 option.
What version of JBoss AS and JVM?
(By the way, I am glad you are no longer hosting the web site on XP - having more than 10 concurrent connections on XP violates the license agreement.)