Error message - core media plugin

I have had some instances on start-up of this message ---------

"Finder quit unexpectedly (or sometimes when trying to access System prefs. "System preferences quit unexpectedly) - while using the Core Media plugin"

-- and cancelling the message just brings it back again in a few seconds. This on a Mac Mini with Mavericks OS. I then have to shut down the Mac and try again as the only way of getting a usable machine.

Any suggestions as to cause ?