Just to close the loop.

I unregistered msxml6.dll from the directory c:\windows\system32 and then registered msxml4.dll in the same directory (using Lyndel's instructions) and the Pronto editors now work.

I want to mention that there are as many as two dozen msxml*.dll files on my drive running from versions 1 up through 6 that are located in several different directories. Only the files located in the windows\system32 directory affected the Pronto editors.

I think this one deserves to be in the FAQs.