hab mal ein bißchen rumprobiert, könnte es an den diversen 100%-Angabe gelegen haben?:
<html>
<head>
<style type="text/css">
body {margin-right:0px;
}
.mainmenu {padding-top:150px;
width:30%;
font-family:Times New Roman;
}
iframe {position:absolute;
top:29px;
left:200px;
width:70%;
height:80%;
border-left:1px #000000 solid;
border-top:1px #000000 solid;
padding-bottom:29px;
}
.submenu {position:absolute;
letter-spacing:0.3em;
top:0px;
left:200px;
height:20%;
}
</style>
</head>
<body scroll="no">
<iframe frameborder="0" src="test.txt" name="main" scrolling="no">
</iframe>
<div class="mainmenu">
<a href="test.txt" target="main">Link 1</a><br>
<a href="test.txt" target="main">Link 2</a><br>
<a href="test.txt" target="main">Link 3</a><br>
<a href="test.txt" target="main">Link 4</a><br>
<div class="submenu">
<a href="test.txt" target="main">Test</a> <a href="test.txt" target="main">Test</a> <a href="test.txt" target="main">Test</a>
</div>
</body>
</html>
ausprobiert mit IE und Firefox