[p3aul]

Well this may be a case of the blind leading the blind here. Bob clearly has a lot more experience with Calibre than I do, but I think he is using a different OS than we are. I also have WinXP pro and when I first installed Calibre I created a folder for my library called Epub. Calibre transferred each file I added to this in a subfolder with the author's name. If there were more than one book by the same author, it got its own subfolder. Thats why you have to be careful to always spell the author the same way. Anyway that why I said do a search in windows for the folder named for an author you know you had in the old library. That will lead you to the name of your initial library folder.
Oh I should add that the root library folder(whatever you named it has the file metadata.db This is the database file Calibre looks for. To be honest with you I don't know how you would get Calibre to recreate this file if you have to create a new library folder. It seems to be a SQLite db file