Thank you all for your suggestions. This hard drive is three years old and appears to be operating nominally and within normal limits. I do share, however, your sense of foreboding! We're backed up to thumb drives and The Cloud (Microsoft OneDrive.)
jimtherev - the only AV we run currently is Windows Defender, which, knock on wood, seems to be effective.
BFM - see paragraph one concerning back ups. When our computer's seven-year-old hard drive crashed, we were NOT fully backed up. We'll not make that mistake again!
Repos - excellent link! I'll give it a try and report back.
Gratefully,
Ed