I think I know what happened.
I had Norton 360, but it expired and I didn't have the money to renew it. So I downloaded the trial version of AVG 9, but I didn't uninstall Norton. AVG probably quarantined Calibre.
Anyway, I decided to uninstall AVG and buy Kaspersky instead.
Kaspersky is installed, Calibre is repaired.
The problem is that Kaspersky classified Calibre as riskware. So far it is working. We will see if it still is tomorrow....
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