Over the past few months, I've come across a few threads of people having problems installing Dropbox. A common feature seems to be that they subsequently can't update anything!
Here's the latest: http://askubuntu.com/questions/55049...able-to-update
So I come across this thread - http://ubuntuforums.org/showthread.php?t=2278384 - and post a response as do several others. The thread is clearly about the confusion between the Unity desktop and the desktop that what see when we install the "next" ISO image.