Isto funciona e já testei:
Observe que: a IFRAME tem como id="iframe_1" e no evento onChange() da Select tem como _target = "iframe_1".
Para testar, crie 3 páginas: estoque.htm vendas.html comercial.html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"
http://www.w3.org/TR...ml4/loose.dtd"><html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script language="javascript">
<!--
function MM_jumpMenu(targ,selObj,restore){ //v3.0
eval(targ+".location='"+selObj.options[selObj.selectedIndex].value+"'");
if (restore) selObj.selectedIndex=0;
}
//-->
</script>
</head>
<body>
<form name="form1" method="post" action="">
<select name="select" onChange="MM_jumpMenu('
iframe_1',this,0)">
<option value=""> </option>
<option value="estoque.htm">Estoque</option>
<option value="vendas.html">Vendas</option>
<option value="comercial.html">Depto Comercial</option>
</select>
</form>
<iframe src="" id="
iframe_1">
</iframe>
</body>
</html>