I'm guessing something about your contents within one of those divs. Is there something thats to wide in #content div?
This worked for me in IE 8's computability mode, with dummy one line content:
<style type="text/css">
#ctmsContainer {
margin: 5px 10px;
width: 100%;
}
#nav {
width: 230px;
float: left;
}
#content {
margin-left: 240px;
}
</style>
<body>
<div id="ctmsContainer">
<div id="nav">
here is my nav
</div>
<div id="content">
here is some content
</div>
</div>