Hi. This appears to have been a "goof up" on my part. I think it stemmed from a glitch that occurred on my computer when I installed Visual Studio 2013 Community and wound up reinstalling it in place (as opposed to an uninstall/reinstall).
Anyway, I wound up doing a serious cleanup - purging several old versions of Visual Studio and in fact ditching Visual Studio 2013 and moving to Visual Studio 2015.
I'd just like to report that all is well now.
Chris - with regard to your response, I'd like to acknowledge that a "Repair" option IS available for the Netduino SDK. However it appears (for me) only after I choose Uninstall from Programs & Features. I was confused by this initially, as I find that many other programs expose "Change", "Repair" & "Uninstall" right from the programs list.
Finally, I see no reason for this topic to remain on the Netduino forums. Maybe it could be purged? Don't want to "clutter up" the Internet...Go to the full post