<div style="width:100%">
  <div style="float:left; background:#f5f5f5; border-right:1px solid black; height:170%; width:120px;"></div>
  <div style="margin-right:2px;margin-top:15px; margin-bottom:5px; width:100%; border:1px solid #f5f5f5"></div>
</div>

.container {
    width: 1024px;
    display: block;
}

#containing_div {
    width: 200px;
    overflow: auto;
}

#container {
    min-width: 1000px;
    _width: 1000px; /* This property is only read by IE6, which gives a fixed width */
}

<div id="container" style="width:100%">
    <div id="primaryNav" style="float:left; width:150px; background-color: Orange">someNav</div>
    <div id="content" style="margin-left: 10px; background-color: Red; overflow: auto;">
        loadsOfSuperInterestingContentI'mSuperSerious<br/>
        <br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/>
        <br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/>
        <br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/>
        Seriously
    </div>
</div>