I mean it looks like your missing your closing </head> from the code.
Code:
<?xml version="1.0" encoding="utf-8"?>
<!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" lang="en" xml:lang="en">
<head>
<link rel="Stylesheet" type="text/css" href="Stylr.css" />
<title>Wifee.co.uk - Your Sims Resource</title>
<script src="glassbox/glassbox.js" type="text/javascript"></script>
</head>
<body><center>
<div id="myGlassboxContent">
hi
</div>
<script type="text/javascript">
var myGlassBox = new GlassBox();
myGlassBox.init( 'myGlassboxContent', '400px', '300px', 'auto' );
myGlassBox.apos( '290px', '35px' );
</script>