Ajax有时无效解决办法

JS:  var XMLHttp = false;
        try
        {
            XMLHttp = new ActiveXObject("Msxml2.xmlHTTP");
        }
        catch(e)
        {
            try
            {
                XMLHttp = new ActiveXObject("Microsoft.xmlHTTP");
            }
            catch(e2)
            {
                XMLHttp = false;
            }
        }
        if(!XMLHttp && typeof XMLHttpRequest!= null)
        {
            XMLHttp = new XMLHttpRequest();
        }
        if(!XMLHttp)
        {
            alert("失败");
        }
        function Verify()
        {
        debugger;
            var name = document.getElementById("txtbAccount").value;
            if(name.length==0){   
                alert('对不起，用户名不能为空!'); 
            }
            else{
                var url = "AppendUserRole.aspx?account="+name+"&rnd="+Math.random();
                XMLHttp.open("GET",url,true);
                XMLHttp.send(null);
                XMLHttp.onreadystatechange =check_Name;
            }
        }
        function check_Name()
        {
            if(XMLHttp.readystate == 4)
            {
                if(XMLHttp.responseText == "true")
                {
                    alert("用户名存在..");
                }
            }       
        }
Page_Load():
if (Request.QueryString.Count > 0)//判断用户是否存在
                {
                    if (Request.QueryString["account"] != null)
                    {
                        string name = Request.QueryString["account"].ToString();
                        if (GovernUserRolesHelper.IsUserExist(name))
                        {
                            Response.Write("true");
                        }
                        else
                        {
                            Response.Write("false");
                        }
                        Response.Flush();
                        Response.End();
                    }
                }
分析：其实不是IE问题，问题来自于由于每次生成的URL都是一样,因为Get请求很特殊，没有返回内容(其实就是为了触发一下某个事件，在回调函数中调用另一个请求来刷新页面)，所以IE自作主张的把这个请求返回的空页面给cache掉了。

解决办法：在生成URL的时候在后面跟加了一个随机数"这样IE这个SB不会在cache掉页面了吧""""
var url = "AppendUserRole.aspx?account="+name+"&rnd="+Math.random();