function obtenerXHR() {
req=false;
if (window.XMLHttpRequest){
req = new XMLHttpRequest();
}else{
if(ActiveXObject){
//definimos un vector con las distintas posibilidades
var vectorVersiones=["MSXML2.XMLHttp.5.0","MSXML2.XMLHttp.4.0","MSXML2.XMLHttp.3.0","MSXML2.XMLHttp","Microsoft.XMLHttp"];
//lo recorremos e intentamos instanciar cada uno
for (var i=0; i<vectorVersiones.length;i++){
try{
req=new ActiveXObject(vectorVersiones[i]);
return req;
}catch(e){}
}
}
}
return req;
}