ie6 footer at bottom problem

I need my footer to stay at the bottom all of the time. Right now, the footer is always at the bottom when the page loads, but if the user expands the page (like they add content via javascript-related buttons) then the footer stays in it’s original place and does not move down. This is the setup and css:

<div id=”container_div”>
<div id=”content_div”></div>
<div id=”footer”></div>
</div>

#container_div {
position: relative;
min-height: 100%;
width:960px;
margin:auto;
background:#FFF;
border-left:3px #CCC solid;
border-right:3px #CCC solid;
}
#content_div {
padding-bottom:50px;
width:100%;
clear: both;
}
#footer {
width:960px;
height:40px;
position:absolute;
bottom:0px;
margin:auto;
padding-top:10px;
}

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