You should use "Microsoft.XMLHTTP" in IE7 and "XMLHttpRequest" in other browsers:
Replace "
objXMLHttpRequest = new XMLHttpRequest();
" with these lines:
if (window.ActiveXObject)
objXMLHttpRequest = ActiveXObject("Microsoft.XMLHTTP");
else if (window.XMLHttpRequest)
objXMLHttpRequest = new XMLHttpRequest();
else
{
alert("Your browser doesn't supoort AJAX");
return null;
}