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Andrew7 - add in 2.79 working OK here.
Try a fresh download from
here and a fresh install - i.e uninstall what's there via Control Panel then do the install. You won't lose settings or library(ies), but if you're concerned you might, back 'em up first, see ==>>
How do I backup calibre?.
If that doesn't work, try a system file check with the scannow option.
BR