Your contact us page and the help center are the only blown out page so the problem lies in formatting of those pages.

Did you add a un needed style there? Cut the content out of those pages and paste in a text file. Then save your zen cart page and test in the browser. I think you will find your issues there

You have 2 stylesheets and you need to choose one of them and trun the other into a .txt for testing.

If you are going with a 760 wide page you should also change this:
.header_wrapper {
width: 100%; /should be 760px/*/
background-color: #cacaca;
}