Unless you've been modifying things in the registry by hand, you should just need to do the opposite of what this FAQ (on opening everything in a single window) tells you to do:
[Always re-use a single window instead of opening new windows)
i.e. Set the first two options shown to Open the Default Lister and clear the Open external folders in a new tab checkbox.
PS: I moved this thread to here form the Screenshots forum since it's not about screenshots.