Ok, the only "wild guess" I can come up with: Did you download calibre from the official download website at
http://calibre-ebook.com/download? Or did you get it from some other download location?
If you obtained it from the official download website, have you tried uninstalling calibre, rebooting your computer and re-installing? (Note: I'm not saying you have to do this now, just wondering if you already have already tried this.)