function GetXMLHttp() {  
    if(navigator.appName == "Microsoft Internet Explorer") {  
    xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");  
    }  
    else {  
    xmlHttp = new XMLHttpRequest();  
    }  
    return xmlHttp;  
    }  
      
    var xmlRequest = GetXMLHttp();      

   function abrirPag(valor){  
    var url = valor;  
      
    xmlRequest.open("GET",url,true);  
    xmlRequest.onreadystatechange = mudancaEstado;  
    xmlRequest.send(null);  
      
    if (xmlRequest.readyState == 1) {  
    document.getElementById("conteudo_mostrar").innerHTML = "<img src='loader.gif'>";  
    }  
      
    return url;  
    }  
      
    function mudancaEstado(){  
    if (xmlRequest.readyState == 4){  
    document.getElementById("conteudo_mostrar").innerHTML = xmlRequest.responseText;  
    }  
    }