Thanks Aaron. I did try your suggestion but it did not work - I even tried putting the new DWORD under both of the nodes. I'm running under 32bit Windows.
From another source I got a work-around that works: when I open up the Account or Opportunity through Outlook, and it opens up the form in the cut-down IE window, hit CTRL-N to open the form again in a full IE window. Because of the way IE duplicates the URL, rather than just opening a new blank window, I get a fully featured IE window with working spell checking. Seems like the original form displayed by Outlook is some cut-down version without Add-ins - maybe its a security "feature"? Unfortunately, by implementing the spell checking as an Add-in, that is getting disabled too!
Thanks, Dave.