function getXmlHttp ()
{
  var xhr = null;
  if (window.XMLHttpRequest) {
    xhr = new XMLHttpRequest ();
  }
  else if(window.ActiveXObject)
  { // Internet Explorer
    try {
      xhr = new ActiveXObject("Msxml2.XMLHTTP");
    } catch(e) {
      try {
        xhr = new ActiveXObject("Microsoft.XMLHTTP");
      } catch(e1) {
        xhr = null;
      }
    }
  }
  else
  {
    alert("Votre navigateur ne supporte pas les objets XMLHTTPRequest...");
  }
  return xhr;
}