This is a very outdated table-based layout, and a pretty messily coded one at that. First and foremost, I would urge you not to use this at all but to find another sample layout - one that is based on correctly used HTML (or XHTML) and CSS. This is damn near impossible to work with in the long run.
If you're still dead set on using this layout, I did manage to center it with some trouble. It's an ugly solution, but that's to be expected when manipulating a layout like this. It still needs some adjustments, but it's too time demanding to fix every detail.
I still say you should scrap this and get a good sample layout instead. If not - hope this helps.
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