simple expandable vertical menu, java script problem

Try this :

<script type="text/javascript">
    $(document).ready(function($) {
        $('#access li').has('ul').addClass('has_submenu');
        $('#access ul li.has_submenu > a').click(function(event) {
            event.preventDefault();
            $("#access ul ul").toggleClass("showHide");
        }); 
    });
</script>

See event.preventDefault in the jQuery API.