I seem to have solved the problem. After more research, I did the following: 1) unplugged the iMac power cord and then plugged it back in and 2) rebuilt the launch services database using Cocktail. Things appear to be back to normal. Thanks for your attention.
Thanks for keeping us posted; I hope your fix sticks.
For future reference, it will be in your and everyone else's best interests if you try fixes one-at-a-time, rather than in gangs; that way you'll know precisely which one worked.
