What are you trying to do by the way? You can easily check on the server side as the page is being displayed whether the session has expired and redirect. You can't abandon from the client but you can redirect to a page with server side script that will abandon.

Code:
<SCRIPT language="JavaScript">
window.parent.parent.location.href='default.asp';
</SCRIPT>
This will also break you out of any frames and redirect to default.asp. If you put a session.abandon at the beginning of the page you redirect to, it should do the trick. Keep in mind that the session timeout timer is restarted essentially any time the browser hits the server for anything so your countdown timer on the page is only valid while that page is up.