This is the URL that requires authorization:
http://c1.microsoft.com/c.gif?DI=4050&PS=81555&PI=40472&TP=http%3a%2f%2fwww.microsoft.com&RF=
It's referenced in this block of Javascript:
<script type="text/javascript">
<!--
var msnGif='<'+'img width="0" height="0" alt="" src="http://c1.microsoft.com'+'/c.gif?DI=4050&PS=81555&PI=40472&TP=http%3a%2f%2fwww.microsoft.com&RF='+escape(window.document.referrer)+'">';document.writeln(msnGif);
//-->
</script>