Well, it looks as though my MSXML files have gotten somehow corrupted. Although diagnostics says they're OK, several programs I use (including, unfortunately, HeroLab) say that they're not and are not working correctly.
I've got a few more things to try to get them fixed, but if they don't pan out I'll be spending significant time making sure everything's backed up, then doing a clean install of Windows (and a lot of praying). SO, if I disappear from the boards for a day or two (I surely hope that's all it ends up taking) that's where I'll be.