Al final me ha salido de esta manera :
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<script language="JavaScript1.2">
function obtenirPosicioAbsoluta(element)
{
var r = { x: element.offsetLeft, y: element.offsetTop };
if (element.offsetParent) {
var tmp = obtenirPosicioAbsoluta(element.offsetParent);
r.x += tmp.x;
r.y += tmp.y;
}
return r;
}
function mostrar(c) {
var i;
document.getElementById(c).style.display='';
ventana = window.frames["iframe"];
sel = ventana.document.getElementsByTagName("SELECT")
var p = obtenirPosicioAbsoluta(document.getElementById(c)) ;
var EX1 = p.x;
var EX2 = document.getElementById(c).offsetWidth + EX1;
var EY1 = p.y;
var EY2 = document.getElementById(c).offsetHeight + EY1;
for(i=0;i<sel.length;i++)
{
var poscombox = obtenirPosicioAbsoluta(sel[i]);
var EXPCB1 = poscombox.x;
var EXPCB2 = sel[i].offsetWidth + EXPCB1;
var EYPCB1 = p.y;
var EYPCB2 = sel[i].offsetHeight + EYPCB1;
if ((EXPCB1 > EX2) || (EXPCB2 < EX1) || (EYPCB1 > EY2) || (EYPCB2 < EY1))
{
sel[i].style.visibility = "visible";
}
else
{
sel[i].style.visibility = "hidden";
}
// sel[i].style.visibility = 'hidden';
}
}
function ocultar(c) {
var i;
document.getElementById(c).style.display='none';
//Para poder acceder a los elementos que se encuenta en el iframe
ventana = window.frames["iframe"];
//Recorremos todos los campos que sean del tipo select
sel = ventana.document.getElementsByTagName("SELECT")
for(i=0;i<sel.length;i++)
{
// se muestra todos los combos
sel[i].style.visibility = 'visible';
}
}
</script>
<head>
<title>Untitled</title>
</head>
<body>
<table>
<tr>
<td><img src="logo.gif" width="78" height="18" alt="" border="0" onclick="mostrar('form');"></td>
</tr>
</table>
<div id="form" style="position:absolute; left:280px; top:25px; width:350px; height:300px; z-index:1; background-color:yellow;display:none" onmouseover="ocultar('form');">
</div>
<!--
<div id="form" style="position:absolute; left:560px; top:325px; width:350px; height:300px; z-index:1; background-color:blue;display:visible" onmouseover="ocultar('form');">
</div>
-->
<iframe frameborder="1" width="800" height="200" align="middle" vspace="100" name="formulario" src="formulario.html" id="iframe">
</iframe>
</body>
</html>