Cambie la ultima tabla por una anchura del 50%, y el problema persiste.
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/DTD/strict.dtd">
<html>
<table style="HEIGHT:12%;WIDTH:100%;" border=0 valign="top" align ="center">
<td align ="left" valign="top">
<table style="HEIGHT:6%;WIDTH:40%;" border=0>
<td align ="left">
<br>
Gracias<b><u> laura </b></u>por iniciar sesion. </td>
</table>
</td>
<td align ="right" valign="top">
<table style="HEIGHT:6%;WIDTH:40%;" border=0>
<td align ="right">
<FORM METHOD = "POST" ACTION="logout.php"><br><br>
laura | <input type="hidden" value="Cerrar sesion" >
<a href="#" onclick = "document.forms[0].submit()">Cerrar Sesion</a>
</FORM>
</td>
</table>
</table>
<br>
<table style="HEIGHT:12%;WIDTH:100%;" border=0 valign="top" align ="middle">
<td align ="middle" valign="top">
<font color=blue>PERFIL</font><br><br>nombre: laura<br><br>localidad: bcn<br> </td>
</table>
<table border="0" align ="middle" cellspacing ="5" cellpadding ="5">
<tr>
<td align="middle">
<form action="envioVideos.php" method="post" enctype="multipart/form-data">
Archivo: <input name="fichero" type="file" maxlength="150">
<br> Nombre: <input name="nombre_archivo" type="text" maxlength="70">
<input name="submit" type="submit" value="Upload!">
</form>
</td>
</tr>
</table>
<br>
<html>
<head>
</head>
<body>
<br><br><table summary="quien sabe" style="width:50%; text-align:center; margin:auto; border:1"><object width="380" height="260">
<param name="movie" value="http://www.youtube.com/v/YeJAA7TQlJ8&hl=es&fs=1>
</param><param name="allowFullScreen" value="true"></param>
<embed src="http://www.youtube.com/v/YeJAA7TQlJ8&hl=es&fs=1" type="application/x-shockwave-flash" allowfullscreen="true" width="380" height="260>
</embed>
</object><tr><td align="center"></td><object width="380" height="260">
<param name="movie" value="http://www.youtube.com/v/YeJAA7TQlJ8&hl=es&fs=1>
</param><param name="allowFullScreen" value="true"></param>
<embed src="http://www.youtube.com/v/YeJAA7TQlJ8&hl=es&fs=1" type="application/x-shockwave-flash" allowfullscreen="true" width="380" height="260>
</embed>
</object><td align="center"><br/></td></tr></table><br><table summary="quien sabe" style="width:50%; text-align:center; margin:auto; border:1"><object width="380" height="260">
<param name="movie" value="http://www.youtube.com/v/-trsqnY3ulo&hl=es&fs=1>
</param><param name="allowFullScreen" value="true"></param>
<embed src="http://www.youtube.com/v/-trsqnY3ulo&hl=es&fs=1" type="application/x-shockwave-flash" allowfullscreen="true" width="380" height="260>
</embed>
</object><tr><td align="center"></td><object width="380" height="260">
<param name="movie" value="http://www.youtube.com/v/-trsqnY3ulo&hl=es&fs=1>
</param><param name="allowFullScreen" value="true"></param>
<embed src="http://www.youtube.com/v/-trsqnY3ulo&hl=es&fs=1" type="application/x-shockwave-flash" allowfullscreen="true" width="380" height="260>
</embed>
</object><td align="center"><br/></td></tr></table><br><br><table summary="quien sabe" style="width:50%; text-align:center"><tr><td align="center"><span class="paginacion">1</span> ][ <a class="paginacion" href='/contingut.php?_pagi_pg=2'>2</a> ][ <a class="paginacion" href='/contingut.php?_pagi_pg=3'>3</a> ][ <a class="paginacion" href='/contingut.php?_pagi_pg=2'>></a> ][ <a class="paginacion" href='/contingut.php?_pagi_pg=3'>Última ] </a></td></tr><tr><td align="center"><br />del <b>1</b> al <b>2</b> de <b>6</b></td></tr></table></body>
</html>