AJAX nao funciona no internet explorer

ajax = new ActiveXObject("Microsoft.XMLHTTP");

var req=null;
var console=null;
var READY_STATE_UNINITIALIZED=
var READY_STATE_LOADING=1;
var READY_STATE_LOADED=2;
var READY_STATE_INTERACTIVE=3;
var READY_STATE_COMPLETE=4;
function sendRequest(url,params,HttpMethod){
if (!HttpMethod){
HttpMethod="GET";
}
req=initXMLHTTPRequest();
if (req){
req.onreadystatechange=onReadyState;
req.open(HttpMethod,url,true);
req.setRequestHeader
("Content-Type", "application/x-www-form-urlencoded");
req.send(params);
}
}
function initXMLHTTPRequest(){
var xRequest=null;
if (window.XMLHttpRequest){
xRequest=new XMLHttpRequest();
} else if (window.ActiveXObject){
xRequest=new ActiveXObject
("Microsoft.XMLHTTP");
}
return xRequest;
}

function OpenAjaxPostCmd(pagina,camada,values,msg,divcarga,metodo,tpmsg) { 
	if(document.getElementById) {
		var ajax = openAjax();
		if(tpmsg=='1'){
			var exibeLoading = document.getElementById(divcarga);
		}
		var exibeResultado = document.getElementById(camada);
		if(metodo=='1'){
			ajax.open("POST", pagina, true);
			ajax.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
			ajax.setRequestHeader("Cache-Control", "no-store, no-cache, must-revalidate");
			ajax.setRequestHeader("Cache-Control", "post-check=0, pre-check=0");
			ajax.setRequestHeader("Pragma", "no-cache");
			valor = CpForm(values)
		}else{
			valor = null
			
			ajax.open("GET", pagina + values, true);
			ajax.setRequestHeader("Content-Type", "application/x-www-form-urlencoded"); 
		}
		ajax.onreadystatechange = function() {
			if(ajax.readyState == 1) {
				if(tpmsg=='1'){
					exibeLoading.style.display = 'inline';
					exibeLoading.innerHTML = msg
				}else{
					exibeResultado.innerHTML = msg
				}
			}
			if(ajax.readyState == 4) {
				if(tpmsg=='1'){
					exibeLoading.innerHTML = ""
					exibeLoading.style.display = 'none';
				}else{
					exibeResultado.innerHTML = ""
				}
				if(ajax.status == 200) {
					var resultado = null;
					resultado = ajax.responseText;
					//resultado = resultado.replace(/\+/g," ");
					//resultado = unescape(resultado);
					exibeResultado.innerHTML = resultado;
				} else {
					exibeResultado.innerHTML = "<br / ><br / ><center>consulta nao pode ser realizada...</center><br / ><br / > <center>" + resultado + "</center>";
				}
			}
		}
		ajax.send(valor);
	}		 
}