Well, can't really offer anything concrete, but first, try to unstuff and install the demo again. See if that works. If not, turn all your unneeded extensions and control panels off and see if that works. And then, if you're really being upset by its refusal to run, try a system software reinstall. Or rebuild the desktop or something. You know, all those oft-discussed Mac fix-alls that people do whenever their computer starts to mess with them
Zap the PRAM! Boot from a CD! Wheeee!
Or, better yet, if you have OS X, try to run it under the new OS for a bit. At least the error code should change. Hope any of my above non-helpful suggestions works.