This div has been centered within your browser window. If you grow/shrink the window, it should adjust to the screen. There are only two div's to pull this off: a wrapper and the container. These two divs work together to position the container div square in the middle. If you are interested in the technique, it is a modification of Matt Taylor's Centered Menus trick.
If you are not to crazy about IE6, you can even use this to keep it positioned center all the time. Click the button below to see what I mean.