[Idoine]

Are you on Vista ?
I had a problem, too, first time I tried Calibre.
I solved it with 2 tricks (better more than less !)
1. Install calibre with a right click on the exe file, then Run as Administrator
2. Launch calibre with the same method.

Edit : BTW, my metadata.db is in Program Files\calibre\bin, though I don't remember if it was a personal choice...