Try this:
CSS:
#content {
margin: 0 auto;
border: 1px solid red;
width:800px;
position:absolute;
bottom:0px;
top:0px;
overflow:auto
}
HTML:
<body>
<div id="content">
...content goes here...
</div>
<body>
If you don't like absolute positioning in this case, just can play with making a parent and child div to this one, both with position:relative
.
EDIT: Below should work (I just put the css inline for the moment):
<div style="margin:0 auto; width:800px; height:100%; overflow:hidden">
<div style="border: 1px solid red; width:800px; position:absolute; bottom:0px; top:0px; overflow:auto">
...content goes here...
</div>
</div>