As we've seen in windows each file type has a default association in the registry, a program that is used to open the document when you double click on its icon. Filesystem folders are also considered documents and by default they open with windows explorer. You won't change the folder association easily to a different program, at least not through control panel (notice how most buttons are disabled in folder options applet).

For many versions I resisted the pressure to make xplorer² the default folder application, but in the end with version 1.72 there is an option to replace windows explorer if you tick the relevant checkbox during the installation process. I don't understand the need for this tweak as I do all my file management through xplorer². It has so many ways to find folders quickly with breadcrumbs, bookmarks etc that I keep it open all the time and open all folders from within it.

But it takes all sorts to make a world and some users find such system integration to their liking, e.g. using Start > Run to open folders or some other program launcher; such people will like the new filesystem folder association with xplorer². The process is simple: tick the installer checkbox to make xplorer² default for your folders, and uninstall it to remove the association, restoring windows explorer. Here are a few clarifications:

• Only filesystem folders are associated.
  xplorer² can read virtual folders but cannot be started automatically when you click on "My Computer" due to a technicality (DDE conversation). Most other folders are taken over.
   
• A single pane xplorer² opens when you click on a folder.
  This way it feels more like windows explorer. Under the hood the installer adds an "action verb" for folders called open_x2 and uses command line switches to open it in single pane mode. To minimize interference with your normal xplorer² window, a special layout x2solo is used too. If you don't like this behaviour and you want to use the same kind of window in all cases, fire up regedit and remove /1 /T and /S:x2solo options from the definition of HKEY_CLASSES_ROOT\directory\shell\open_x2\command as in this snapshot . If you want to reuse an open xplorer² window have a look at bSingleWindow registry tweak in registry.txt
   
• I don't like xplorer² opening folders like that.
  If you change your mind and wish to have windows explorer back, the simplest solution is to uninstall xplorer² then reinstall it without ticking the folder association checkbox. Alternatively use this registry script to keep open_x2 without having it the default verb. This script has to run in administrator mode; if you are logged as a normal user, use run as to start an elevated xplorer² and double click on the REG file from within it.

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