this is an often asked question, closing a browser (client side) does not automatically destroy the session (server side) its page currently holds
there is no good solution to solve your problem, waiting for IE7 to provide a window.closing event, :-)
but for now, you can try something like this, add a javascript variable to your page, set it to false when you click on buttons or links, or do postbacks,
<script language="javascript">
var bClosing = true;
function window.unload()
{
if(bClosing)
window.open("quit.aspx","_blank");
}
</script>
<a href="..." onclick="bClosing=false;">...</a>
in quit.aspx, call Session.Abandon(), in your Session_End in global.asax, remove the 用户ID from Application["ID"]