It probably isn't working because of the createXMLHttpRequest() function. Try...

Code:
function createXMLHttpRequest(){
  var xmlHTTP;
  try{xmlHTTP = new XMLHttpRequest();}
  catch(e){
    try{xmlHTTP = new ActiveXObject("Msxml2.XMLHTTP");}
    catch(e){
      try{xmlHTTP = new ActiveXObject("Microsoft.XMLHTTP");}
      catch(e){
        alert("Your browser does not support AJAX!");
        return false;
      }
    }
  }
  return xmlHTTP;
}
Your code could use some clean up. Check out my AJAX object for more help.