var http = crearAjax();

function crearAjax()
{
    var xmlhttp=false;
     try 
    {
      xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
    } 
    catch (e) 
    {
        try 
        {
               xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
          } 
        catch (E) 
        {
               xmlhttp = false;
          }
     }

    if (!xmlhttp && typeof XMLHttpRequest!='undefined') 
    {
          xmlhttp = new XMLHttpRequest();
    }
    return xmlhttp;
}