function advertisement() {
    // Array de imagenes
    var images = new Array
    images[1] = "http://www.westlaw.aranzadi.es/banners_productos/pt/1.gif"
    images[2] = "http://www.westlaw.aranzadi.es/banners_productos/pt/2.gif"
    images[3] = "http://www.westlaw.aranzadi.es/banners_productos/pt/3.gif"
    images[4] = "http://www.westlaw.aranzadi.es/banners_productos/pt/4.gif"
    
    // Array de enlaces
    var links = new Array
    links[1] = "pt/marketing/spa/novedades_contrata_spa.html"
    links[2] = "pt/marketing/spa/busqueda_universal_spa.html"
    links[3] = "pt/marketing/spa/mejora_actu_spa.html"
    links[4] = "pt/marketing/spa/mas_contenido_spa.html"
    
    var randomnumber = Math.floor(Math.random()*(images.length-1)) + 1;
    document.getElementById('advertisement').innerHTML = '<a href="' + links[randomnumber] + '"><img src="' + images[randomnumber] + '" border="0" alt="Westlaw Contrata"></a>';
}