addShutdownHook is in java.lang.Thread packaged in rt.jar. add this file and tools.jar to your JBOSS_CLASSPATH.
hope it works.
I had JBoss running then I loaded IPlanet and WebSphere on the same machine and JBoss suddenly had the identical error to what you describe. Check to make sure that you are using a jdk 1.3.1 (in your classpath "java -version"), JBoss requires it. Somehow mine was reset to jdk1.2.2. I just added the path ...jdk1.3.1\bin\java into the run.bat file (last line) and everything is back up.