var page = "system/check_username.php";
var target = "user_check";
function checkUsername(url,getstr,target)
 {
    // native XMLHttpRequest object
   document.getElementById(target).innerHTML = 'Loading...';
   if (window.XMLHttpRequest) {
       req = new XMLHttpRequest();
       req.onreadystatechange = function() {usrCheckDone(target);};
       req.open("POST", url + "?getstr=" + getstr, true);
       req.send(null);

   // IE/Windows ActiveX version
   } else if (window.ActiveXObject) {
       req = new ActiveXObject("Microsoft.XMLHTTP");
       if (req) {
           req.onreadystatechange = function() {usrCheckDone(target);};
           req.open("POST", url + "?getstr=" + getstr, true);
           req.send();
       }
   }
         //setTimeout("ajax(page,'user_check')", 20000);
}

function usrCheckDone(target) {
   // only if req is "loaded"
   if (req.readyState == 4) {
       // only if "OK"
       if (req.status == 200 || req.status == 304) {
           results = req.responseText;
           document.getElementById(target).innerHTML = results;
       } else {
           document.getElementById(target).innerHTML="ajax error:\n" +
               req.statusText;
       }
   }
}