woudn't putting this in the code be better:

if(typeof(XMLHttpRequest) == "undefined")){
	XMLHttpRequest = function(){
		var xmlHttp = null;
		try
	    {
			xmlHttp	=	new ActiveXObject("Msxml2.XMLHTTP");
	    }
		catch (e)
	    {
			xmlHttp	=	new ActiveXObject("Microsoft.XMLHTTP");
	    }
		return xmlHttp;
	}
}

so later instead of using this.

var xhr = window.ActiveXObject ? new ActiveXObject("Microsoft.XMLHTTP") : new XMLHttpRequest();

you use this

var xhr = new XMLHttpRequest();