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") {

empa = new Image();
empa.src = "images/bot01_in.gif";
empb = new Image();
empb.src = "images/bot01.gif";

sera = new Image();
sera.src = "images/bot02_in.gif";
serb = new Image();
serb.src = "images/bot02.gif";

dela = new Image();
dela.src = "images/bot03_in.gif";
delb = new Image();
delb.src = "images/bot03.gif";

cona = new Image();
cona.src = "images/bot04_in.gif";
conb = new Image();
conb.src = "images/bot04.gif";

homa = new Image();
homa.src = "images/bot_home_in.gif";
homb = new Image();
homb.src = "images/bot_home.gif";

infa = new Image();
infa.src = "images/bot_mail_in.gif";
infb = new Image();
infb.src = "images/bot_mail.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");
}
}