function xmlhttp() {		//crear el objeto de ajax
	var xmlhttp;
	try { xmlhttp = new ActiveXObject("Msxml2.XMLHTTP"); }
		catch (e) { try { xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); }
		catch (e) { try { xmlhttp = new XMLHttpRequest(); }
		catch (e) { xmlhttp = false;}}}
	if (!xmlhttp) return null;
	else return xmlhttp;
}