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Pagali- it could be interference from 3rd party program.
First have a look at the following to see if anything rings any alarms ==>>
calibre freezes/crashes occasionally?
Assuming nothing rdoes, try running the viewer directly in Safe Mode - if that works you'll have to figure out the culprit via a process of elimination. Keyboard and clipboard enhancers/gadgets are the sort of things that might create problems.
Failing that try doing a
System File Check
Running sfc /scannow has been known to fix APPCRASH errors.
Ctrl+C in the Viewer works for me on Windows 7 and Windows 10 - can't imagine why it shouldn't work for you on 8.1.
BR