If you update calibre while calibre is open, you can trigger having to restart your computer so that files that were in use can be overwritten. In my experience, that has been the only time that I need to restart my computer after a calibre installation.
As for @Cactus Chef's wife's issue, I would suggest making sure that no calibre processes are running, uninstalling calibre, deleting the C:\Program Files\Calibre2 directory and any subdirectories and then re-installing calibre. This should not affect your calibre configuration or libraries.
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