Hi,
Thanks for your reply!
Deleting the metadata folder was a suggestion to remedy a similar problem, think it was in case the .db file was corrupt. Just thought it would be worth a try - thanks for the info on restoring it.
I've just tried deleting the calibre profile - I'm using Windows 7 so this was in a slightly different place than XP - C:\users\appdata\roaming\calibre. Then uninstalled calibre and reinstalled v7.43. Installed and opened correctly (I noticed the difference when I reinstalled the new version with the profile deleted, it asked me to setup my reader/library details again).
Then I restarted my computer and tried to open Calibre - only to get the same error message as before. Is there anything else I can try? I really, REALLY don't want to lose Calibre