XMLHttpRequest, document element DOM parsing

XMLHttpRequest use, works only if url is on same site.
checklogin.php give <KO/> if login è fails
<OK>XXX</OK> with XXX id of user when login è is ok

var loginOK = false;

var httpRequest;

function validateLogin()
{
 var login = document.getElementById("NomeForm");
 var logintxt = login.value;
 var password = document.getElementById("PasswdForm");
 var passwordtxt = password.value;
 var url = "/forum/checklogin.php?username="
 + logintxt + "&password=" + passwordtxt;

 if (window.XMLHttpRequest) { // Mozilla, Safari, ...
    httpRequest = new XMLHttpRequest();
    if (httpRequest.overrideMimeType) {
       httpRequest.overrideMimeType('text/xml');
    }
 }
 else if (window.ActiveXObject) { // IE
    try {
       httpRequest = new ActiveXObject("Msxml2.XMLHTTP");
    }
    catch (e) {
       try {
          httpRequest = new ActiveXObject("Microsoft.XMLHTTP");
       }
       catch (e) {}
    }
 }

 if (!httpRequest) {
    alert('Giving up :( Cannot create an XMLHTTP instance');
    return false;
 }
 httpRequest.onreadystatechange = loadLogin;
 httpRequest.open('GET', url, true);
 httpRequest.send(null);
}

function loadLogin()
{
  if (httpRequest.readyState == 4) {
      if (httpRequest.status == 200) {
         response = httpRequest.responseText;
	 if (response == "") {
	   alert ('login/password errate.');
	 } else {
	   var responseXML = httpRequest.responseXML.documentElement;
	   user_id = responseXML.firstChild.nodeValue;
	   LinkSitoForm = document.getElementById('LinkSitoForm');
	   LinkSitoForm.setAttribute('value', 'http://forum.cellularmagazine.it/profile.php?mode=viewprofile&u=' + user_id);
	   NomeForm = document.getElementById('NomeForm');
	   PasswdForm = document.getElementById('PasswdForm');
	   PasswdForm.setAttribute('enabled','no');
	   var whchis = document.getElementById("trcheckpasswd");
	   whchis.innerHTML = "OK"
	 }
      } else {
         alert('There was a problem with the request.');
      }
   }
   else {
   }

}