Plain javascript XMLHttpRequest implementation.

generic-xhr.js

function xhr(){ 
    try {
        return new XMLHttpRequest();
    }catch(e){}
    try {
        return new ActiveXObject("Msxml3.XMLHTTP");
    }catch(e){}
    try {
        return new ActiveXObject("Msxml2.XMLHTTP.6.0");
    }catch(e){}
    try {
        return new ActiveXObject("Msxml2.XMLHTTP.3.0");
    }catch(e){}
    try {
        return new ActiveXObject("Msxml2.XMLHTTP");
    }catch(e){}
    try {
        return new ActiveXObject("Microsoft.XMLHTTP");
    }catch(e){}
    return null;
}

function ajaxRequest(url, callback, postData) {
    var req = xhr();
    if (!req) return;
    var method = (postData) ? "POST" : "GET";
    req.open(method, url, true);
    // req.setRequestHeader('User-Agent','XMLHTTP/1.0');
    if (postData)
        req.setRequestHeader('Content-type','application/x-www-form-urlencoded');
    req.onreadystatechange = function () {
        if (req.readyState != 4) return;
        if (req.status != 200 && req.status != 304) {
            return;
        }
        callback(req);
    }
    if (req.readyState == 4) return;
    req.send(postData);
}