I've put together a sample that also explains why your script failed. The videobox zip file was extracted into the folder that contains the html file. If you have another setup you'll have to adjust the script and css paths:
<html>
<head>
<meta content="text/html; charset=ISO-8859-1" http-equiv="content-type">
<title>My page loads a video!</title>
<script type="text/javascript" src="./video/js/mootools.js"></script>
<script type="text/javascript" src="./video/js/swfobject.js"></script>
<script type="text/javascript" src="./video/js/videobox.js"></script>
<link rel="stylesheet" href="./video/css/videobox.css" type="text/css" media="screen">
</head>
<body scroll="no"
style="background-image: url(BG.jpg);"
onload="doOpenVideoDelayed('http://www.youtube.com/watch?v=uhi5x7V3WXE', 'Your cool video!', 'vidbox 800 600 play', 1000);">
<div style="height: 100%; width: 100%;">
</div>
</body>
<script type="text/javascript">
function doOpenVideoDelayed(url, caption, options, delay)
{
window.setTimeout( function() {
Videobox.open(url, caption, options);
},
delay
);
}
</script>
</html>
Hope this still helps!
Best regards,
Manfred