I just resetted my preferences to the default and have found that my default file explorer has been changed to DOpus. Even when I click on My Computer, it runs DOpus. I swear I didn't touch any of the change default file explorer setting.
Can I have DOpus as my default file explorer but still have Windows Explorer when I click on My Computer?
If you reset Preferences to defaults it uses a fixed default configuration, ignoring the 5 or so options you get to set during the initial installation wizard. That's probably why the setting was changed even though you've never changed it yourself. (Well, technically you did change it, in the wizard. 
)
It's easy to change back. Go to Preferences / Miscellaneous / Explorer Replacement.
You can select Don't Replace Explorer if you always want Explorer to open when you double-click on My Computer and folders in other programs and the desktop.
Or you can select Replace Explorer for all but the following folders and then specify My Computer in the list below so that double-clicking folders opens Opus but double-clicking My Computer still opens Explorer.
I figured that out thanks =)