IIRC - there was a spate of corrupt downloads a few years ago, one theory as to cause was that one of the major US based ISPs was caching and disdribuling corrupt copies.
The OP should cross-check the checksums of downloaded .msi against values available from the official download pages,
From
here it appears there are about 25,000 people using calibre 5.14 on Windows. Many (most ?) would use Defender.
BR