Required fields check

Hi,

I have a problem on a website before making the purchase. There is a form and you have to fill the shipping information. But, when you do not fill one of the required fields, and press ‘buy button’, the shopping cart page is reloaded.
I need that the errors that are generated by empty required fields occur on the same page of the form, and the cart page will not be reloaded. I have detected this in wpsc-shopping_cart_page.php template. The form is like this:

<form class=’wpsc_checkout_forms’ action='<?php get_option(‘shopping_cart_url’); ?>’ method=’post’ enctype=”multipart/form-data”>