Actually the reason the last thread veered off is because I brought up the GeForce3 suggestion myself, but anyway...
I realize that these problems are sometimes hard if not impossible to diagnose. As far as background programs running, I shut down everything except for Explorer and Systray to test. It still locks up -- so I don't know what else to tell you there.
You are using standard DirectX calls, which is good. But so are many other programs, games and screensavers I have on my system. None of them lock up. Only the Fish screensaver does. I can run all other DirectX screensavers continuously (I have over two dozen of them; I keep collecting them). Some of them, like the Silex "dancing skeleton" screensaver, are pretty taxing to the system, probably moreso than the aquarium. They never lock up. I can also run 3DMark 2001 in looping mode continuously with no problems.
Since the problem only appears in the Fish screensaver, I would tend to come to the conclusion that it is the Fish screensaver that is the source of the problem, at least in reference to this particular PC's configuration.
Anyway, like you said, I'm out of ideas. Guess I just won't run the screensaver on this PC.