There are 2 versions of JBoss-4.2.3 GA. One for JDK6 and one for JDK5. You probably downloaded the JDK6 version which cannot be used with a JDK5. You have two options - either to use JDK6 or to download the JDK5 version of JBoss-4.2.3 GA
Yes you are absolutely right. I grabbed the wrong version (i.e. jdk1.6). That fixed the problem.
Again thanks a lot.