创建xmlhttp对象！

第一步创建xmlhttp对象！

        try
        {
            var xmlHttp = false;
            try
            {
                xmlHttp = new XMLHttpRequest();
            }
            catch
            (trymicrosoft)
            {
                try
                {
                    xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
                }
                catch (othermicrosoft)
                {
                    try
                    {
                        xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
                    }
                    catch (failed)
                    {
                        xmlHttp = false;
                    }
                }
            }
            if(!xmlHttp)
            {
                alert("无法创建 XMLHttpRequest 对象！");
            }

或者  简单..

function createXMLHttpRequest() {
if (window.ActiveXObject) {
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
}
else if (window.XMLHttpRequest) {
xmlHttp = new XMLHttpRequest();
}
}