Windows Explorer double click going up level rather than opening file\folder

I am running Windows 7. For some time now if I double click a file or folder in windows explorer left or right pane it moves up a level to the parent folder and shows the contents of that in the right pane rather that opening the contents of that folder.

I have no trojans or viruses. I am using the QTTabBar.

Any ideas of why this is happening?

Thanks, Tom.