Originally Posted by Mr_Mark
Thanks for your reply. I will keep trying as you suggest. But this has been happening for two days now, and each time my computer freezes I have to reboot it.
How long did you wait?
Like Kovid says, it will probably work OK if you leave it for a while. Calibre does lock your PC for a while when it starts, but that doesn't last very long; here (4GB RAM, 2.4 GHz Core i5) it's something like 15 seconds when loading a 52K books library.