After days of frustrating failed attempts, I sent Stormjumper a private message and his reply solved it. What a great forum and what dedicated, friendly and helpful people here who make it work.
I'm posting his advice for all to see, in case someone else has this problem in the future.
I junked my 11 year-old XP antique for a fast machine with Windows 7 Ultimate. But when I tried to put my Never Expire key into DVDFab in the new machine, it wouldn't take it. (It said thanks, it will register me when DVD restarts, but it still showed 'trial user' over and over again.)
Stormjumper suggested right-clicking on the DVDFab icon and giving it administrator's privileges. And that fixed it!
Mahalo nui loa (Hawaiian for thanks very much)