This is a very annoying issue, and should have been done differently that the way Microsoft designed it.

When working in Outlook Express, you will notice that the layout panels has little [x] on the top right corner of each panel. These are used to close the panel with.

At times, you might want to remove the extra panels, and sometimes you may accidentally click the [x], which will close it when you do not want it to.

Click on the [x] on the top right side of the Folder window and it will disappear.