WordPress jQuery is not defined error

Your primary problem is that you are de-registering core jQuery. Don’t do that. Ever. Period. If you get rid of that, your other scripts will work properly. Just do this:

function wpse62373_register_js() {
    if ( ! is_admin() ) {
        wp_enqueue_script('quicksand', get_template_directory_uri() . '/js/quicksand.js', array( 'jquery' ) );
        wp_enqueue_script('easing', get_template_directory_uri() . '/js/easing.js', array( 'jquery' ) );
        wp_enqueue_script('custom', get_template_directory_uri() . '/js/main.js', array( 'jquery' ), '1.0', TRUE);
        wp_enqueue_script('prettyPhoto', get_template_directory_uri() . '/js/jquery.prettyPhoto.js', array( 'jquery' ) );    
    }

}
add_action( 'wp_enqueue_scripts', 'wpse62373_register_js' );

There are ways to replace core WordPress scripts properly, but I won’t facilitate a bad practice such as replacing a core WordPress script by providing the answer.