If you do a quick search for “IE sidebar down” – there will be hundreds of topics. The answer is always the same:
something in your sidebar or in the content area is too wide for the container div around it.
It could be an image, a long code line, a long link, a not-well-formatted post…
It’s a long trial and error: try to see individual posts and notice when the sidebar goes down and when it doesn’t. Take it from there. if it goes down everywhere – check the sidebar and start removing elements one by one.