I'm having the same problem with the non-functional installation dll. Calibre stopped loading epubs one day and when I reinstalled it Windows told me one of the dll's could not be run. The program still works if I run it as administrator, but every time I load an ebook a stupid Windows message pops up for me to approve the running of the program. This sucks if I open multiple ebooks at a time. I've been using Calibre for nearly two years now with no problems, and I've not installed any new programs (incl. anti-virus programs) recently, so it's very puzzling why it would just stop working one day.
Any ideas for a permanent fix?