As the #mainBanner div is 1024px wide, then you will get a horizontal scrollbar on screen sizes less than around 1040px wide. Simply removing the width setting from #mainBanner would allow it to resize with the screen, and setting a min-width of 900px (same as #wrapper) would prevent it getting narrower.
Also noticed you footer content div is too wide as well - padding adds to the width, so the width should be changed to 880px. #footer could also use the 900px min-width setting.
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