This 500 error is usually the result of folder permissions that are not to the standard your web server requires. Look at the permissions of your root folder and of the zp-core folder as those are most likely the issue. It could also be the permissions of the setup.php script. What the permissions need to be is specific to our provider, but very often if you have loose (0777) permissions you will get this error.
It is also possible that there is some other PHP error occuring so you should check your script error logs. Normally, though, PHP errors result in blank pages, not server errors.