Under Folder options, choose File Folder.
Set it to open with explorer.exe
Set it as the default action.
EDIT - better description ....
Select Tools -> Folder Options, and click on the File Types
tab. Under the Extensions column on the left, scroll down
to the N/A section and find the entry for Folder.
Highlight the Folder entry (not the File Folder entry),
and click on the Advanced button at the bottom right.
In that window you should see the three actions: Cmd Here,
Explore, and Open. Highlight the action Open, and click on
the Set Default button. Click OK through all the windows
you have open, in the process of closing them.
This should reset Open as the default action when you are
double-clicking a folder in the right-hand Explorer window.