Many, many thanks to all who responded. It is indeed an IE7 bug, and I have found a workaround. A reader explained:

There’s certainly nothing wrong with your page; you’ve just run across a troublesome bug with Internet Explorer’s display of floats. The paragraph that wrap around the bottom of the float will cover up anything in the float, preventing you from clicking on links. It doesn’t really have any visual effect, because the background is set to the default transparent, but if, for example, you were to have the style declaration:
p {background-color: white;}
you would see what was happening.
So the trick here is to convince IE that your navigation menu is in front of the narrative. The simplest way to do this is to add one extra property to .navigator’s style:
.navigator { position:relative; }
IE will bring the menu to the front and your links should work in IE. That shouldn’t have any other effect on the display of your page.
I hope that helps. Let me know if you have any other questions.

Well, I do have other questions — like, why is Bill Gates so rich, turning out crud like this? But many, many thanks to that reader, and to all others who took the time. I tried the suggested solution, and it works.