If it's a clean install, why don't you trash it and re-upload it and start over? That's probably a LOT faster than wasting a lot of time debugging what's broken.
Or you can fiddle around with figuring out what might be broken in the autoloader process by adding a new file at /admin/includes/extra_configures/test_autoload.php containing just the following:
Code:
<?php define('DEBUG_AUTOLOAD', true);
Maybe that'll show you where execution stops or what files it fails to load properly.
I suspect wiping it and doing a clean install is much simpler.