Good question. Unfortunately IE 7 does the exact same thing for me. I checked in all the settings, as well as patches, and the only thing I could find was this:
With IE7 more HTML compliant it's changing the layout. You may have to adjust your HTML some. Are you creating it based on IE7, a previous IE, Firefox, or something else? It might depend on what the new forum layout was designed for.
Hope this helps Ziggy. I'll ask around for any more details.