When you click on the Advanced Search popup help button, the popup that is displayed is missing its [Close] button. Here's an updated copy that corrects the issue in file /includes/templates/template_default/popup_search_help/tpl_main_page.php, with changes highlighted in red:
 * Override Template for common/tpl_main_page.php
 * @package templateSystem
 * @copyright Copyright 2003-2005 Zen Cart Development Team
 * @copyright Portions Copyright 2003 osCommerce
 * @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
 * @version $Id: tpl_main_page.php 2870 2006-01-21 21:36:02Z birdbrain $
<body id="popupSearchHelp" onload="resize();">
<div><?php echo TEXT_SEARCH_HELP ?></div>
<p><a href="javascript:window.close();"><?php echo TEXT_CURRENT_CLOSE_WINDOW; ?></a></p>