Well I noticed that I had a notification in my Intel Rapid Storage Technology Center.  I cleared it (set to be treated as "Normal"), then it stayed normal until I tried to start Media Center.  So it's a bad hard drive (BIOS diagnostic agrees).  It was a Seagate Barracuda, and I've replaced it with a SSD---spending all evening starting from 8.0 from a recovery drive, doing all the updates, then upgrading to 8.1, then the Pro Pack to get Media Center back (is there any better way to run a TV tuner? please?).  So five hours later, it works.