woocommerce/templates/checkout/form-login.php

<?php
/**
 * Checkout login form
 *
 * @author 		WooThemes
 * @package 	WooCommerce/Templates
 * @version     1.6.4
 */

if ( ! defined( 'ABSPATH' ) ) exit; // Exit if accessed directly

if ( is_user_logged_in() ) return;
if ( get_option('woocommerce_enable_signup_and_login_from_checkout') == "no" ) return;

$info_message = apply_filters('woocommerce_checkout_login_message', __( 'Already registered?', 'woocommerce' ));
?>

<p class="woocommerce_info"><?php echo esc_html( $info_message ); ?> <a href="#" class="showlogin"><?php _e( 'Click here to login', 'woocommerce' ); ?></a></p>

<?php woocommerce_login_form( array( 'message' => __( 'If you have shopped with us before, please enter your details in the boxes below. If you are a new customer please proceed to the Billing &amp; Shipping section.', 'woocommerce' ), 'redirect' => get_permalink(woocommerce_get_page_id('checkout')) ) ); ?>