Me sigue sin funcionar, mira:
title>Documento sin título</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<script language="Javascript">
function mostrar(nombreCapa){
document.getElementById(nombreCapa).style.display= "";
}
function ocultar(nombreCapa){
document.getElementById(nombreCapa).style.visibili ty="hidden";
}
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</head>
<body background="../../imagenes/FondoderechaEventos.png" onLoad="MM_showHideLayers('Layer3','','show')">
<div id="Layer1" style="position:absolute; left:100px; top:51px; width:98px; height:149px; z-index:1" align="center"><img src="http://www.forosdelweb.com/imagenes/mapas/MapaEventosGral.jpg" width="418" height="520" border="0" >
</div>
<div id="capa1" style="position:absolute;width:100px;height:26px;t op:167px;left:251px;" onmouseout="ocultar('capa2')" onmouseover="mostrar('capa2')"></div>
<div id="capa2" style="position:absolute;width:100;height:100;top: 112px;left:556px;display:none">prueba</div>
</html>