I ran chkdsk with the /f, /r and /b switches to be as thorough as possible and a ton of index entries got deleted, although no bad sectors were found. Still couldn't boot, though. Continuously launching the Startup Repair utility off the Win 7 installation disc then finally fixed the problem, I think I had to attempt 4-5 times before it got nailed, for whatever reason.
The system is now booting fine, but I want to find out what might be causing these problems. It's not the first time that chkdsk had to make a lot of corrections. It has happened several times recently, so I am suspecting that the HDD is dying. It is slightly over 5 years old, after all.
I'll try running memtest86+ overnight again, even though I just did it recently. If that doesn't turns up anything, I'll try Seatools once more, even though I, like memtest, ran that very recently, too.