http://blogs.msdn.com/jscript/archive/2007/08/12/windows-script-5-7-released-for-windows-2000-windows-xp-and-windows-2003-server.aspx
I suspect this means that someone running IE6 might now also have JS 5.7. In this case your sniff code won't define the needed ctor.
f2富二代官网入口
objREQ = window.ActiveXObject == false ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP") ;