function ajaxRequest(url,capa,previ,posterior) {
   											var AJAX = null;
   											if (window.XMLHttpRequest) {                     
      									AJAX=new XMLHttpRequest();                    
   											} else {                                         
      									AJAX=new ActiveXObject("Microsoft.XMLHTTP");  
   											}                                                
   											if (AJAX==null) {                                
      									alert("El navegador no admet AJAX.");  
      									return false                                  
   											}
   											AJAX.open("GET", url, true);
   											AJAX.onreadystatechange = function() {                      
      									if (AJAX.readyState==4 || AJAX.readyState=="complete") { 
         								document.getElementById(capa).innerHTML =previ+AJAX.responseText+posterior;
      									}                                                        
   											}
   											AJAX.send(null);                                              
												}