body {background-color:#000000;
background-image:url('back.jpg');
background-repeat:no-repeat;
background-attachment:fixed;
background-position:center;
}

#principal {background-color:#; margin:auto; width:1000px;}

#contenu {
float:left;
position: relative;
  top: 1em;
  left: 2.5em;

}
#contenubase{
float:left;
 position:relative;
top: 6em;
left: -57.9em;
}
#contenuindex {
float:left;
position: relative;
  top: 1em;
  left: 2.5em;
overflow-x: hidden;
overflow-y: auto;
}

#nav ul {list-style:none;}
#nav ul li {float:left;}

<style type="text/css">
img, div { behavior: url(iepngfix.htc) }
</style>