This is my HTML code:

<div id="main" align="center">
<div id="container">
<div id="topbanner"><img src="images/lifestyle_banner.jpg" width="793" height="87" alt="Lifestyle Arts and Crafts Ltd" border="0" /> </div>

<div id="leftnav"><img src="images/nav_left_spacer.jpg" width="190" height="16" alt="" border="0" /></div>
<div id="navigation">&nbsp;</div>

<div id="leftnav" align="left"><!-- Begin Left Nav -->
leftnav
</div><!-- End Left Nav -->

<div id="maincontent" align="left"><!-- Begin Main content -->
main content
</div><!-- End Main content -->
</div>
</div>

and this is my CSS:

body {margin: 0px; padding: 0px; }
div {margin: 0px 0px 0px 0px; }
/* DIV Styles */
#main { width: 100%; margin:0px; padding: 0px; }
#container { width:793px; margin:0px; padding: 0px; }
#topbanner { width:793px; height:87px; margin: 0px; padding: 0px; }
#maincontent { width: 603px; float:left; margin:0px; padding: 0px; }
#leftnav { width: 190px; float: left; margin:0px; padding: 0px; }
#navigation { width: 603px; float:left; background-color: #BBC57D; height: 16px; margin:0px; padding: 0px; }