//得到XMLHTTP实例
function GetXmlHttp() {
    var xmlHttp;
    try {
        // Firefox, Opera 8.0+, Safari
        xmlHttp = new XMLHttpRequest();
    } catch (e) {
        // Internet Explorer
        try {
        xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
         } catch (e) {
         try {
             xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
         } catch (e) {
         alert("您的浏览器不支持AJAX！");
         return;
            }
        }
    }

    return xmlHttp;
    
}