OK, just to explain. The folders on the Index page which have a Cross on them indicate that the Forum has Sub-Forums.
Inside the forum, the threads which have a Dot on the folder Icon represent threads which you have posted in. I only implemented that a few months ago, jus so people can get a better idea as to what they have posted in. I know it's not very clear from just looking at the folder.....maybe I need to make some better folder icons??