Per poter centrare perfettamente un movie devi creare una tabella in percetuale alla pagina.
Questo è lo script corretto, modifica solo il nome dell'swf e le sue dimensioni.

codice:
<html>
<head>
<title>Titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>

<body bgcolor="#bababa" leftmargin="0" topmargin="0" marginwidth="0" marginheight="0">
<table width="100%" height="100%" border="0" cellpadding="0" cellspacing="0">
  <tr>
    <td width="100%" height="100%" align="center" valign="middle"><object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,29,0" width="616" height="472">
        <param name="movie" value="sito.swf">
        <param name="quality" value="high">
        <embed src="sito.swf" quality="high" pluginspage="http://www.macromedia.com/go/getflashplayer" type="application/x-shockwave-flash" width="616" height="472"></embed></object></td>
  </tr>
</table>
</body>
</html>