function GetXMLHttp() {
if(navigator.appName == "Microsoft Internet Explorer") {
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
}
else {
xmlHttp = new XMLHttpRequest();
}
return xmlHttp;
}

var xmlRequest = GetXMLHttp();

outro:
instrução.js
function abrirPag(valor){
var url = valor;

xmlRequest.open("GET",url,true);
xmlRequest.onreadystatechange = mudancaEstado;
xmlRequest.send(null);

if (xmlRequest.readyState == 1) {
document.getElementById("meio").innerHTML = "<img src='loader.gif'>";
}

return url;
}

function mudancaEstado(){
if (xmlRequest.readyState == 4){
document.getElementById("meio").innerHTML = xmlRequest.responseText;
}
}