IE is interpreting the combined width of your content area and sidebar area as too big for the parent container. When this happens the browser (in this case IE) causes the element on the right (in this case the sidebar) to appear below other elements.
There are lots of ways to fix this but in the end you’ll most likely need to decrease the width of your content area or sidebar. Alternatively you can increase the width of the parent container (usually called wrap, wrapper, content or something like that).
How you do this is the real question. You may want to make sure you code is standards compliant first, you may want to include an IE only style sheet, or if the content areas are only off by 4-5 pixels you may just want to make the changes in the themes style.css.
Hope the helps.