IE issue with floats

[nloding]

OK, here's my basic setup:

<div id="wrapper">
<div id="menu">blah blah</div>
<div id="right-nav">blah blah</div>
<div id="content">blah blah</div>
</div>

This is meant to be a fixed-width three column layout. Now, I'm new at this so, here's my issue. With the following CSS coding, it shows up correctly in Mozilla/Netscape, but not IE. In IE, the "content" div is below the content of the left and right-nav divs.

#wrapper {
margin-left: auto;
margin-right: auto;
width: 800px;
}

#menu {
float: left;
width: 200px;
}

#right-nav {
float: right;
width: 200px;
}

#content {
background: url(blah.jpg) repeat-y;
width: 400px;
margin-left: 200px;
margin-right: 200px;
}

I've tried playing with margins, but nothing helps. It just sits way down there in IE. I can't figure it out; mostly because I look at CSS coding for other sites, and it works perfectly with THE SAME CODE! I'm lost.

Any hints? Suggestions? Ideas how to figure out what is going on?