function maxPantalla() {
    /* Maximiza la ventana */
    var tw = top.window;
    tw.moveTo(0, 0);
    if (document.all) {
        tw.resizeTo(screen.availWidth, screen.availHeight);
    }
    else
        if (document.layers || document.getElementById) {

        if (tw.outerHeight < screen.availHeight || tw.outerWidth < screen.availWidth) {
            tw.outerHeight = tw.availHeight;
            tw.outerWidth = tw.availWidth;
        }
    }
}

/* Centra un elemento en la pantalla */
/* elemento : lo que quiero centrar */
/* offset cuanto se desplaza en x e y*/
function centrar(elemento, offsetX, offsetY) {
    var Alto = 0;
    var elem = document.getElementById(elemento);
    var divisorX = 1;
    var divisorY = 1;    
    if (offsetX == null) {
        var divisorX = 2;
        var offsetX = 0;
    }
    if (offsetY == null) {
        var divisorY = 2;
        var offsetY = 0;
    }
    /*  document.getElementById('cargando').style.visibility = 'hidden';*/
    if (document.getElementById && !document.all) // Detect FF
    {
        var IpopTop = (window.innerHeight - elem.offsetHeight) / divisorX;
        var IpopLeft = (window.innerWidth - elem.offsetWidth) / divisorY;
        if (divisorY == 2)
            Alto = elem.offsetHeight;
        var Ancho = elem.offsetWidth;
        elem.style.top = IpopTop + Alto - offsetY + 'px';
        elem.style.left = IpopLeft - offsetX + 'px';
    }
    else // Just assume otherwise (IE) at this stage.
    {
        var IpopTop = (document.body.clientHeight - elem.offsetHeight) / 2;
        var IpopLeft = (document.body.clientWidth - elem.offsetWidth) / 2;
        elem.style.left = IpopLeft + document.body.scrollLeft + 'px';
        elem.style.top = IpopTop + elem.style.offsetHeight + document.body.scrollTop + 'px';
    }
    /*   document.getElementById('cargando').style.visibility = 'visible'; */
}