bbPress.org

Looks fine in Firefox 3 or IE7 on Windows XP.

Does it happen with the stock theme?

Does your HTML validate?

http://validator.w3.org/check?verbose=1&uri=http%3A%2F%2Fwww.watchandtrain.com%2Fdogforums%2F

My guess is that it’s improperly marked up code causing it to do that to you. For example, you are using an unordered list <ul> in the sidebar, but you’re not using any list items <li>. That will surely cause problems. I’d fix the invalid markup before trying to fix this specific layout issue.