A follow-up, as I appear to have fixed the problem.
The problem was with Cygwin, though I never had it installed. I sometimes run the X-Live CD (http://xlivecd.indiana.edu/
), and this uses a version of Cygwin. It does not install (does not appear in Add/Remove Programs), but apparently makes registry changes that Arduino doesn't like.
I cleared all registry entries that had to do with Cygwin using a registry cleaner, and once I did that, the Arduino environment starts with no errors and I was able to interface with my board.
I replicated this on another computer to make sure that it was the problem, and my suspicions were confirmed.
Thank you for your help on this.