You're having real problems with this IE conditional comment, aren't you? ;-)
Try changing:
<!--[if IE 7]>
<link rel="stylesheet" href="<?php bloginfo('stylesheet_url'); ?>" type="text/css" media="screen" />
<link rel="stylesheet" href="<?php bloginfo('template_directory'); ?>/ie7.css" media="screen" type="text/css" />
<![endif]-->
to
<!--[if IE 7]>
<link rel="stylesheet" href="<?php bloginfo('template_directory'); ?>/ie7.css" media="screen" type="text/css" />
<![endif]-->
in header.php. When that's done, try adding something like:
#nav_menu {position:relative;top:30px;}
to ie7.css and see if that helps.