If default wp_head cause including old JQuery version such as 1.4.2. Add following into function.php to De-register current default JQuery and include new one (version 1.7.2) in this case.
<?php
// code to include in theme's functions.php
if( !is_admin()){
wp_deregister_script('jquery');
wp_register_script('jquery', ("http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"), false, '1.7.1');
wp_enqueue_script('jquery');
}
?>