ANSWER:

The 500 error may be caused because the user running C:Program Files (x86)PHPphp-cgi.exe doesn’t have proper permissions on the directory containing your PHP files.

1. Check the service manager to see which user is running this process.
2. Check the security ACL for the PHP directory and files.
3. Ensure that the user in (1.) has read/execute and list directory permissions and propagate these to all files.

by leftcase from http://serverfault.com/questions/381882