You haven't told us the version of JBoss and Java you are using. But from what i have seen in the forums, you seem to be using JDK1.6 version of JBoss-4.2.3 GA with an incompatible JDK (i guess JDK1.5).
Tip#1: See this thread for more details http://www.jboss.com/index.html?module=bb&op=viewtopic&t=143822
Tip#2: First start the server from the console and ensure that everything works fine. Once you are sure that there are no issues with the downloaded server, you can then try starting it from the IDE. This way you can narrow down the problem to the IDE or the server installation itself.
You are D MAN! Thank you very much.