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Different browsers may use different default values for the margin and padding settings in both HTML tables and in CSS. A first step might be to explicitly set all the margins and paddings to zero.
If it is IE that is behaving differently, do a Google search for CSS Bugs in IE and you'll find a wealth of resources. Also, ensure that your DOCTYPE is set to a valid setting - some browsers go into "quirks mode" when the DOCTYPE is not a modern and syntactically correct version.
DerekH