Do you have many programs running continuously in the system tray, try closing those that provide an exit/close option in their right click menu.
Have you tried running Calibre in Safe Mode -
http://windows.microsoft.com/en-us/w...t-is-safe-mode
Easy way to start it
http://www.howtogeek.com/howto/windo...ng-the-f8-key/
If it works OK in safe mode then its almost certainly a conflict - a process of elimination is the best way to find it - but tedious.