Open folder in Explorer mode by Default

* Open My Computer window.
* Click Tools, Folder Options, then click File Types.
* From the list, locate the entry FOLDER with extension of (NONE).
* Click Advanced, select Explore, then Set Default
* Click OK and close the dialog.

OR

To accomplish this via the Registry:

* Open Regedit.exe and navigate to: HKEY_CLASSES_ROOT\Folder\shell

* In the right pane, set the value of (default) to explore