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Gandalf31
Have a look in the Manual, starting here -
http://manual.calibre-ebook.com/faq.html#miscellaneous
Its probably another program 'interfering', some are identified in the manual. Most anti-virus programs are actually OK these days, unless you have very aggressive settings.
Its almost certainly another program in your start up, possibly one that you don't even know is there. The only way to find the 'culprit' is via a process of elimination.
Sometimes the 'culprit' program will have a place where you can exclude other programs or directories from its operations - e.g Roboform. Sometimes you don't need it to run all the time anyway - eg the Logitech configuration utility can be run from the control panel when you need it, but given half a chance it will inveigle its way into your startup programs and run all the time.
Please let us know the outcome, so that the manual can be updated if you find something new.
BR