I've never used that site before, so maybe I'm way off, but it just looks like a simple case of your content's too wide.
The div container is the div in which all the other divs are placed, correct? You gave that fixed width 960px. But then that center div's content is wider than the 540px width you have set (the image is 640px), plus 170px times 2 for the sidebars, plus padding...so in the end the browser, when rendering the page, is squishing it all into that 960px when obviously there's more than 960px there.
Would defining a horizontal overflow for that middle div work?