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").innerHTML = "<img src='loader.gif'>";  
}  
return url;  
}  
function mudancaEstado(){  
if (xmlRequest.readyState == 4){  
document.getElementById("conteudo").innerHTML = xmlRequest.responseText; 
}
}