Steve:
Thanks for the reply. I am on a MacBook Core 2 Duo, 2GHz.
Relaunching the finder does, indeed, get rid of the problem for the time being. Then it recurs, and this can happen at any time (though I'll start looking more closely at what's running when it does).
No, on the new software additions and yes, I did repair disk permissions.
Let me know if you think of anything and, again, thanks.