Thanks, Ath; I agree it is better to solve the root problem. However, that is beyond my abilities. And I am using the most recent, stable release. I suspect that part of the problem is Eudora is an old 32-bit pre-Vista program (last updated in 2006), whereas I am using it on Win7/64. It is no longer being developed.
However, I also suspect that there is something more fundamental going on. Eudora causes the same problem on another Win7/64 machine, so it is not specifically the Eudora installation on my machine. I also experienced it in RightNote -- whenever I viewed any html page it had captured and stored as a note internally, CPU usage shot up to ~25%, and returned to 0% as soon as I switched to a non-html note in the program; then back up to 25% when viewing any html-based note.
So I agree that something is not being processed correctly, whether in certain email messages in Eudora, or html-based notes in RightNote. I suspect that troubleshooting it is not a trivial task, and as I say, it is beyond my abilities.