（1） 写发送代码
var myXmlHttpRequest = "";
myXmlHttpRequest = getXmlHttpRequest();
            if (myXmlHttpRequest) {  //xmlHttpRequest创建成功了，才能发送请求
                //地址一定要写正确
                var url = "../zhuCe/zhuCeYanZheng.aspx?username=" + $('Text1').value;
                myXmlHttpRequest.open("get",url,true);
                myXmlHttpRequest.onreadystatechange = getData;
                myXmlHttpRequest.send(null);
            }

（2） 写接受数据代码：
function getData() {
            if((myXmlHttpRequest.readyState == 4) && (myXmlHttpRequest.status == 200)){
                //alert("nihao");
                var returnMessage = myXmlHttpRequest.responseText;
                //alert(returnMessage);
                if (returnMessage == "用户名不正确"){
                    alert("aaa");
                }
            }  
        }
(3)另外需要加两个函数
function getXmlHttpRequest() {
            var xmlHttpRequest;
            if(window.ActiveXObject){
                xmlHttpRequest = new ActiveXObject("Microsoft.XMLHTTP");
            }else{
                xmlHttpRequest = new xmlHttpRequest();
            }
            return xmlHttpRequest;
        }
       
function $(id) {
            return document.getElementById (id);
        }