Yes, that is the log I was (trying to) referring to. I'm at work right now, and I guess I should have written down the entire log event line. All log entries/actions were successful except this single event, when the scan for updates actually begins.
I have tried several fixes, including a patch from MS.
Apparently there are two essential Certificates required, and I have them both -- something like "MS Root Authority" and a "No Liability" from someone else (something like Verifax..?) --- this one is expired, but the MS reference I have says that it should still work anyway. I'm not sure if this situation is related at all to my current problem.
I guess it's time for a new computer. I'm reluctant to buy one without knowing exactly what MS's forthcoming OS will require to run well. I've heard that Dell will be implementing some new hardware features this fall, probably intentionally in sync with the availability of the new OS.