The chances are that there are permissions issues on the Calibre library files. This is a frequent problem when one does an upgrade from Vista to Windows 7 in that it leaves files under the Documents and Settings area as read-only and Calibre needs full access.
What I would suggest is:
- Copy your existing Calibre library folder to a new location.
- Right-Click the Calibre Library folder and ask for its properties. Then ensure that the permissions are set to allow anyone full control. Make sure you tick the box for the permissions to be applied to all sub-folders and contained files.
- Relaunch calibre and tell it to use the new location for the Calibre library.
Hopefully that will resole your problem.