Ok I just uninstalled the JEMS installed jboss (using the uninstaller).
I then tried reverting back to the standard "extract zip file and run" type jboss install.
This fails to load properly too now. Same errors.
Now I'm totally stumped. Could it be an issue with my registry? I'm using Windows XP.
I restarted windows and now it works.
I have no clue why.
Lesson learnt: when all else fails, try rebooting.