Computer refurb update
Scrubbed the drive and started installing Windows from scratch. The installer did some extensive disk repairs first. Then on one of the restarts after installation, it kacked, tried more repairs, then kacked again.
I thought this was just another case of Windows rot, but now I smell a bad hard drive. Will replace it and try again.
Knock on wood, but this would be the first time a drive ever failed on me, rather than just getting replaced because it got too small.