Help with hiding a div

Hi folks,

I have a javascript / css code on my cart page to hide/show the credit card info to the user (using a dropdown box).
The problem is that if a user submits the form after choosing ‘credit card / nab’ and leaves the page… If they use their back button or go back to the cart the payment type is already selected and cannot be changed to another option!!

How can I fix this?

-------Header -----------

<script type="text/javascript"><!--
var lastDiv = "";
function showDiv(divName) {
	// hide last div
	if (lastDiv) {
		document.getElementById(lastDiv).className = "hiddenDiv";
	//if value of the box is not nothing and an object with that name exists, then change the class
	if (divName && document.getElementById(divName)) {
		document.getElementById(divName).className = "visibleDiv";
		lastDiv = divName;

<style type="text/css" media="screen"><!--

.hiddenDiv {
	display: none;
.visibleDiv {
	display: block;


------  Body --------

<option value="">Select a payment method</option>
				<option value='nab' <?php if(isset($_SESSION['BILLING']['PAYMENTTYPE']) && $_SESSION['BILLING']['PAYMENTTYPE'] == 'nab'){echo ' selected="selected" ';} ?>>Credit Card</option>
				<option value='offline' <?php if(isset($_SESSION['BILLING']['PAYMENTTYPE']) && $_SESSION['BILLING']['PAYMENTTYPE'] == 'offline'){echo ' selected="selected" ';} ?>>Direct Deposit</option>
				<option value='paypal' <?php if(isset($_SESSION['BILLING']['PAYMENTTYPE']) && $_SESSION['BILLING']['PAYMENTTYPE'] == 'paypal'){echo ' selected="selected" ';} ?>>Paypal</option>

	<table id="nab" class="hiddenDiv">
				<td align="left" class="text"><span class="brown3">Card Type :</span> </td>
				<td align="left"><select name="cctype" class="username">
				<option value=""></option>
				<td align="left" class="text"><span class="brown3">Credit Card Number :</span> </td>
				<td align="left"><input type="text" name="ccnumber" size="24" value="%%CCNUMBER%%" class="username">
				<td align="left" class="text"><span class="brown3">Expiration Date (mm/yyyy) : </span></td>
				<td align="left"><select name="ccmonth" class="username">
				<option value="">Select Month</option>
				<select name="ccyear" class="username">
				<option value="">Select Year</option>
				<td align="left" class="text"><span class="brown3">CVV Code :</span> </td>
				<td align="left"><input type="text" name="cvvcode" size="5" value="%%CVVCODE%%" class="username">
			<!-- END CREDITCARD -->

Try to force the cache to reload on this page for this script every time the page is loaded.