Perchè non riesco a centrare il quiz nella pagina? Posto il codice:

<!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" xml:lang="it">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>quiz</title>
</head>
<body>
<center>
<script language="javascript">
<!--
onload=function(){
var lar;
var alt;
var sW=screen.width;
var sH=screen.height;
if(sW==800 && sH==600){
lar = 800;
alt = 600;
}else{
lar = 960;
alt = 720;
}
document.write('<object classid="clsid27CDB6E-AE6D-11cf-96B8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=7,0,19,0" width="' + lar + '" height="' + alt + '" ID="sf" VIEWASTEXT>');
document.write(' <param name="movie" value="quiz.swf" />');
document.write(' <param name="quality" value="high" />');
document.write(' <param name="wmode" value="window" />');
document.write(' <param name="allowScriptAccess" value="always" />');
document.write(' <param name="allowFullScreen" value="true" />');
document.write(' <embed src="quiz.swf" quality="high" name="sf" allowScriptAccess="always" allowFullScreen="true" pluginspage="http://www.macromedia.com/go/getflashplayer" type="application/x-shockwave-flash" width="' + lar + '" height="' + alt + '" />');
document.write('</object>');
}
</script>
</center>
</body>
</html>