I recently bought a new Windows-based computer (with Windows 7) and downloaded the newest edition of ADE. No problems for a few weeks. The last time I downloaded a book (file) to my Nook, I was told it could not be opened because it had been returned (it hadn't). I tried reinstalling ADE and I was going to return the book to the Library and start over. Then I was told the item couldn't be returned because I needed to authorize the computer. When I tried authorizing, I got the message that it is already authorized. I don't know what to try next. Please advise.