var requete = null;

//CREATION D'UN OBJET XMLHTTPREQUEST
try {
  requete = new XMLHttpRequest();
}
catch (essaimicrosoft) {
  try {
    requete = new ActiveXObject("Msxml2.XMLHTTP");
  }
  catch (autremicrosoft) {
    try {
      requete = new ActiveXObject("Microsoft.XMLHTTP");
    }
    catch (echec) {
      requete = null;
    }
  }
}

if (requete == null)
  alert("Erreur HMLHttpRequest");