XHTTP = function(silent) {
  this.transport = this.getXHTTPTransport(silent);
  this.lastResponseText = "";
  this.lastResponseStatus =  0 ;
  this.lastResponseStatusText = "";
}

XHTTP.prototype.getXHTTPTransport = function() {
  var result = false;
  var actions = [
    function() {return new XMLHttpRequest()},
    function() {return new ActiveXObject("Msxml2.XMLHTTP")},
    function() {return new ActiveXObject("Microsoft.XMLHTTP")}
  ];
  for(var i =  0 ; i < actions.length; i++) {
    try{
      result = actions[i]();
      break;
    } catch (e) {}
  }
  if (!result) {
    alert("Error during initializing XMLHTTP");
  }
  return result;
 }

XHTTP.prototype.isEnabled = function() {
  return this.transport;
}

XHTTP.prototype.load = function (url, debug) {
  if (this.transport) {
    this.transport.open("GET", url, false);
    this.transport.send(null);

    this.lastResponseText = this.transport.responseText;
    this.lastResponseStatus = this.transport.status;
    this.lastResponseStatusText = this.transport.statusText;

    if (debug) {
      alert(this.transport.responseText);
    }

    if (this.transport.status !=  200 ) {
      alert(url + " : ERROR " + this.transport.status + " - " + this.transport.statusText);
      return "-1";
    } else {
      var result = this.transport.responseText;
      return result;
    }
  } else {
    alert("Transport is not initialized");
  }
}



var xmlHttp = new XHTTP();
if (xmlHttp.isEnabled()) {
 if (xmlHttp.load("Default2.aspx?x=1&y=2") != "-1") {
  // all ok
...
 }
}