this will show a div in the dead center of the browser:

.myDiv {
position: fixed;
left: 50%;
top: 50%
overflow: auto;
margin-top: -50px;
margin-left: -100px;
background-color: #EEE;
padding: 5px;
border: 1px solid #333;
display: none;
width: 300px;
}

so here is the full code in an example i created:




<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Wallpaperama.com show center Div</title>
<style type="text/css">
.WallpaperamaDiv {
	position: relative;
	left: 50%;
top: 50%;
overflow: auto;
	margin-top: -50px;
	margin-left: -100px;
	background-color: #EEE;
	padding: 5px;
	border: 1px solid #333;
	width: 300px;
}
.main{
	height: 700px;
	width: 700px;
	border: 1px solid #333;
}
</style>
</head>

<body>
<div class="main">
<h1>Wallpaperama!</h1>
<div class="WallpaperamaDiv">This box should be in the center of the browser <br />as you can see from this, it works</div>
</div>
</body>
</html>