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Innula - is there anything in
FAQ - calibre freezes/crashes occasionally? that may be relevant.
In Task Manager's Start-up tab you can Disable the start up programs. I would start by disabling half of them, and restart. If it calibre doesn't work disable the other half. If calibre still doesn't work then... hmmm - drivers, services, have a look to see if there are any 'oddball' services or drivers running.
Also have a look in Control Panel->Programs and Features to see if anything 'unusual' has been installed in recent times. Also check Windows updates, I got an update to my video card s/w the other day - it dumped bloat into startup and installed an unwanted shell menu extension, I gave both of them short shrift.
BR