Ajax 问题啊,高手请进

先看代码如下：  我调试的时候createXMLHttpRequest()老是出现“Access to restricted URI denied"错误
 <script language="javascript" >
       var xmlHttp;
       var flag;
       function createXMLHttpRequest(){
            if(window.XMLHttpRequest){
               xmlHttp=new XMLHttpRequest();
            }
            else if(window.ActiveXObject){
              try{
               xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
               }catch(e){
                  try{
                        xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
                     }catch(e){}
                }  
            }
           
      }
      function checkUserid(userid){
        
         createXMLHttpRequest();
         xmlHttp.open("GET", "checkServlet?userid="+userid,true);
         xmlHttp.onreadystatechange=checkUseridCallback;  
         xmlHttp.send(null);
       
       }
       function checkUseridCallback(){
        
          if(xmlHttp.readyState==4){
             if(xmlHttp.status==200){
                var text=xmlHttp.responseText;
                alert(text);
                if(text=="true"){
                   flag=false;
                   document.getElementById("msg").value="用户名已存在";
             }
             else{
                flag=true;
                document.getElementById("msg").value="可以注册";
              }
           }
        }
        
     }
     function checkForm(){
        
        return flag;
      }
      </script>
      <form action="" method="post" onsubmit="return checkForm()">
       用户名:<input type="text" name="userid" onblur="checkUserid(this.value)"/>
         <span id="msg"></span><br/>
       密&nbsp;&nbsp;码:<input type="password" name="password"/><br/>
         <input type="submit" value="注册" />
       </form>"