Thanks CK. Looking in Update History shows code 0xc19000101. When I tried to download Setupdiag, I got Debugdiag instead which yielded a screen I couldn't figure out what to do with it. Back to more head scratching unfortunately.
Update: Searching found suggestions that this is likely a driver problem. Everythig on this Dell desktop is and has been working with no apparent problems, and I've had no problems running updates through 25300, so I'm still scratching my head.