You would certainly benefit from using a responsive framework for your project. It would save you a good amount of headaches. However, seeing the structure of your HTML I would do the following:
Please check the example: http://jsfiddle.net/xLA4q/
HTML:
<div class="nav-content-wrapper">
<div class="left-nav">asdasdasd ads asd ads asd ad asdasd ad ad a ad</div>
<div class="content">asd as dad ads ads ads ad ads das ad sad</div>
</div>
CSS:
.nav-content-wrapper{position:relative; overflow:auto; display:block;height:300px;}
.left-nav{float:left;width:30%;height:inherit;}
.content{float:left;width:70%;height:inherit;}