First off, you have an extra </header> right before the ad div, and you have a <br /> in the ad. This is directly from your page source:
HTML Code:
</span> </fieldset> </nav> </div> </header> <div id="ad_global_below_navbar"><br /> <table border="0" cellspacing="30" align="center" >
And you have extra </center> and extra </div> tags in there too. I would run the page through an html validator, fix the sloppy html, and then fix the CSS/spacing issues.