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if(xmlhttp.readyState==4)
{
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  if(xmlhttp.status==200)
  {

  }
}

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function check_user(id) {
  if (id == "")
  return;
  document.getElementById("userid_msg").innerHTML = "<img src='images/loading.gif' align='absMiddle'> ¼ì²éÖУ¬ÇëÉÔºò¡­¡­";
  var url = "../RegistValidate.ashx?username=" + escape(id);
  xmlHttp.open("GET", url, true);
  xmlHttp.onreadystatechange = ShowResult;
  xmlHttp.send(null);
  }
  function ShowResult() {
  if (xmlHttp.readyState == 4) {
  if (xmlHttp.status == 200) {
  var s;
  s = xmlHttp.responseText;
  if (s== "OK")
  document.getElementById("userid_msg").innerHTML = "<img src='images/correct.gif' align='absMiddle'>";
  else
  document.getElementById("userid_msg").innerHTML = "<im