After some wide-ranging internet searches, I found out how to uninstall a Windows patch. So with some trepidation I unstalled KB933928.
It worked! Windows Mail is fully functional again! And so far as I can see, the loss of the patch doesn't seem to have caused problems elsewhere (unless anyone knows differently?)
But of course I now have a further worry. How do I stop KB933928 from automatically downloading and installing itself again? I'm sure there's a way of monitoring these patches before they install, but have I got to do that for evermore, to keep KB933928 at bay?