I've taken a look at this: please try the following changes to style.css:
1. In style declaration #header a img change:
margin-left: -360px;
to
margin-left: -230px;

2. Add style declaration:
.primary {
margin: 0 auto;
text-align: center;
}


Basically, this horizontally centres the image and the clickable header links on the web page. The complication is that IE and Firefox are somewhat incompatible in this area and one has to be very careful. Importantly, the above code works when the user changes the viewed font size, and this is a key requirement of any solution. Similarly, it correctly recentres both the image and the links when the browser width is changed by dragging the right window handle. I have tested it with IE 6.0 and Firefox 1.5 and it appears to work for me, though whether it is exactly what you want is another question.