Well, I don't want to "open" the key as such, just to get rid of it, since it's a problem. My logic was: let's assume I had never previously installed ThumbsPlus. In that case there would be no references to it anywhere, including registry keys. Therefore, assuming the offending key is a dedicated key for ThumbsPlus, I'd have thought getting rid of it and starting from scratch should solve the problem. (Although ThumbsPlus is sold as a "service pack" it's a full installer. There is no version 8 for download, just the Service Pack 1 installer.)
When I locate the key within regedit, I can't do anything with it other than export it or edit it. However, as I don't know what values to add to the subkey fields I am stuck.
BTW - I tried to expand another key (for Quicktime) and it gave me the same message/s. It must be something to do with a bad uninstall, I am guessing, as the next Quicktime key is accessible.
Anyway, I am about ready to concede defeat. I will email Cerious and ask them to send me the reg key. If they can be bothered. If I cared more about ThumbsPlus I would restore my system from a backup, but it'd have to be a fairly dated backup and I don't feel like going through the painful process of getting everything else back up to where it is now. I had pretty much stopped using ThumbsPlus anyway (I use Directory Opus now for all my graphics stuff) and the "upgrade" install was just one of those reflex things you do because you've paid for the thing.
Thanks so much for all your help.