Not Responding is sort of normal (for short periods) on my Medium power W7 system.
Calibre has lots of things done in background and the GUI does not always get enough attention
until things slow down.
Even on my old P4 (XP) this only lasts under a minute.
Note: When Queuing a goodly number of books (for conversion/transfer...) there can be a serious lack of feedback UNTIL the queue completes.
One word: Patience
Calibre will let you know if it Barfs