try wrapping everything into a
<div class="wrapper">
...
</div>
container, with the following properties (or you can play with your own):
.wrapper
{
width:1000px;
left:0;
right:0;
margin-left:auto;
margin-right:auto;
}
Next, remove the margin-right
from your nav menu.
The wrapper will act as a margin guide for the rest of the document, so the alignment for the elements will therefore be automatic.