I downloaded "Calibre v1.47.0", and opened the Windows Installer Package normally. I always am able to install software, and this problem met me for the first time. An error message pops up, saying: This installation package could not be opened. Contact the application vendor to verify that this is a valid Windows Installer package. I found some threads, but they seemed to be discussing not being able to open a specific type of file (msi in one case), or specific software (OpenOffice in another). Calibre is an E-Book manager, and it is normal software which I was able to download before. They proposed solutions in those threads which seemed to require time and tasks, so I decided not to try them since my problem isn't exactly theirs. I tried some CMD commands though, which they also proposed, to no avail. In the end I decided to post my own case, and I hope this issue is simple and can be easily solved; at least it seems to be so. Thanks in advance for any help.