Horizontal and vertical center for a div
#centered_div {
position: absolute;
left: 50%;
width: 800px; /* width of your div */
margin-left:-400px; /* half width of your div and make it negative */
top: 50%;
height: 600px; /* height of your div */
margin-top: -300px; /* half heigt of your div and make it negative */
}