function createAjax() {
	var ajax;

	try{
        ajax = new ActiveXObject("Microsoft.XMLHTTP");
	}
	catch(e) {
    	try {
            ajax = new ActiveXObject("Msxml2.XMLHTTP");
		}
		catch(ex) {
        	try	{
            	ajax = new XMLHttpRequest();
			}
			catch(exc) {
			    alert("Esse browser não tem recursos para uso do Ajax");
				ajax = null;
			}
		}
		return ajax;
	}

    var arrSignatures = ["MSXML2.XMLHTTP.5.0", "MSXML2.XMLHTTP.4.0",
	   				     "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP",
						 "Microsoft.XMLHTTP"];
	for (var i=0; i < arrSignatures.length; i++) {
        try {
		    var oRequest = new ActiveXObject(arrSignatures[i]);
			return oRequest;
		}
		catch (oError){
		}
	}

	throw new Error("MSXML is not installed on your system.");
}