<script type="text/javascript" src="js/prototype.js"></script>
<script type="text/javascript"><!--
function pokupka() {
var url = 'http://sub.mydomen.ru/cent.php';
var params = '?cn=1215154';
var ajax = new Ajax.Updater (
{success: 'tccont'},
url,
{method: 'get',
parameters: params,
onFailure: centerror}
);
}
function centerror(request) {
$('tccont').innerHTML = "Ошибка";
}
// --></script>

<form>
<a href="javascript://" onclick="pokupka();">Купить</a>
<div id="tccont"></div>
</form>