Thanks for the reply.
I tried number 1. Trustedinstaller restarted correctly. I waited 90 minutes. KB978251 still failed to install.
Note: the error code I get is "80092004" NOT "_0x_80092004". I found the link you posted on my own, several weeks ago. It seems to be 18 months old, or so. The error code and the reasons prompting the code seem quite different.
2. I ran WU normally yesterday. I ran it in aggressive mode again, after step 1 failed. Again, the update failed after a reboot.
3. I booted with Kaspersky turned off, not uninstalled. I don't want to go through the hassle of reinstalling, since Kaspersky has never prevented any other windows update. Anyway, Kaspersky off, fresh boot, installed KB978251 from a downloaded standalone executable from MS Download Center; fail to install.
What else can I try?