I think its because of the long line in the sidebox: Blue/Turquoise/Aqua (10)
In Firefox and IE7, that hangs of the side of the page, and doesn't increase the width of the sidebox, while in IE6, the sidebox is expanding to contain it, therefore there isn't enough room for the center content (so it shifts down).
My advice is to either find a way to shorten that category name, or tell me you don't want to do that, and you'll have to expand the sidebox in all browsers to avoid that issue. Removing the lines before the subcategories may help as well.
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