nombre = navigator.appName;
version = parseInt(navigator.appVersion);

if ((nombre == "Netscape" && version >= 3) || (nombre == "Microsoft Internet Explorer" && version > 3)) 
ver = "correcta";
        else ver = "incorrecta";

if (ver == "correcta") {



vola = new Image();
vola.src = "images/bot_volver_in.gif";
volb = new Image();
volb.src = "images/bot_volver.gif";

arra = new Image();
arra.src = "images/bot_arriba_in.gif";
arrb = new Image();
arrb.src = "images/bot_arriba.gif";

}

function abajo(imgName) {
        if (ver == "correcta") {
        document[imgName].src = eval(imgName + "a.src");
}
}
function arriba(imgName) {
        if (ver == "correcta") {
        document[imgName].src = eval(imgName + "b.src");
}
}