to start can you please take all 3 files and put it in ONE file to make sure the problem comes from CSS and not from your tables, very often when you play with including headers and footers you have table problems.
If its css, i suggest you play with positions :
from this :
CODE
div.mainnav {
width: 189px;
top: 446px;
left: 19px;}
to this :
CODE
div.mainnav {
position:relative;
float:left;
width: 189px;
top: 446px;
left: 19px;
}
This should make it so the nav and the content area are in the same line but the nav floated left, as well with your content but you set a certain margin to your content so it goes a litle bit on the right (just as your css saids)