Fatal error: installing at WAMP

That error means exactly what it says; PHP is trying to call a function named is_admin() that has not been defined. That function should be defined in the wp-includes/functions.php file. Check that file for the is_admin() function. Either way, I would suggest reinstalling WP. If the file is there, it should be working. If it isn’t there, it needs to be.

URL path is:
http://localhost/wordpress/wp-admin/install.php

FYI: This doesn’t really help troubleshooting as localhost refers to the local computer. In your case http://localhost is your computer, in my case http://localhost is my computer.