Open any Explorer window and select View, Folder Options. Click the File Types tab, and under Registered File Types, select Folder (NOT File Folder). Click the Edit button, and in the Actions box, you'll notice that "open" appears in bold. In this same box, select Explore, then click the Set Default button. Click Close twice, and the next time you double-click a folder, it will open in a two-paned Explorer view. And of course, you can still open a folder in a single-paned window by right-clicking it and selecting Open.

(Note: If you think you might want to revert to the default single-paned view, be sure to read next week's Windows 98 tip before following the above technique. It'll take some Registry editing to get things back to normal.)