I've solved it! Thanks to this post:
http://www.winmatrix.com/forums/inde...howtopic=14341
which referenced
http://windowsxp.mvps.org/slowrightclick.htm,
I determined it was caused by a faulty context-menu handler. By using
ShellExView from NirSoft (
http://www.nirsoft.net/utils/shexview.html), I
determined which context menu was bad -- DiffDoc, in my case. I disabled
that context menu, and shazam! All my shortcuts work properly again.
Bizarre symptoms for a problem like that, but at least now I know how to
fix it if it happens again.
Gary