Firstly, I really like your header. That's a wonderful graphic. There's also some nice use of textured backgrounds on your site.
Looking under the hood, I was initially stunned to see that this was based on the future Zen template. future Zen is intended as an easy way to move away from the tables-based Classic template and harness the power of the near-tableless template_default. You have then done what I love to see most, namely amended it beyond recognition to make something that is unique and your own.
I'm less sure about the reintroduction of those tables, and indeed that's what's caused your problem. Let's pick it apart and provide you with a very easy solution ...
Firstly, the errant behaviour does not maifest itself in all browsers. It's there in Internet Explorer, but everything looks fine in Firefox. Secondly, although the black cell expands and contracts from page to page, it's not the only symptom. Your logo and the search bar also move about and are centered above the black cell, whatever width that might be.
Using the Internet Explorer Development Toolbar to outline the tables on your page shows that the width of the table that you have wrapped around almost everything varies from page to page. This is actually your problem. While by default Firefox expands it to take up as much space as possible, Internet Explorer's default is to expand it only as far as its content demands - hence the variation that you're experiencing.
To resolve this you need to force the width of this table either by giving it a number of pixels or better, since it is constrained by the mainWrapper, by adding the following to your stylesheet> .centershop {width:100%}One small further comment, you have given this table, and I suspect others, a class that is used uniquely for that table. Better practice would be for these to be IDs in which case the CSS needed would be> #centershop {width:100%}