function getXMLHttpRequestObject()
{
  if(window.XMLHttpRequest)
      return new XMLHttpRequest();
  else if(window.ActiveXObject)
      return new ActiveXObject("MICROSOFT.XMLHTTP");
  else
      alert('Your browser does not support Ajax');
}

function makeRequest()
{
  xhr = new getXMLHttpRequestObject();
  var url = "ajax.php?username="+document.getElementsByName('username').value+"&password="+document.getElementsByName('password').value;
  xhr.onreadystatechange = initRequest;
  xhr.open('GET', url, true);
  xhr.send(null);
}

function initRequest()
{
  if(xhr.readyState == 4) {
    document.getGetElementById("responseDiv").innerHTML = responseText.msg;
  }
}