Hi again everyone !
Sorry I totally forgot to give my setup : Windows 7 pro / Clevo M560a (old notebook)/ 4 Go RAM / 20 Go space free, and I got admin rights.
I already tried to execute Calibre install as admin but with no luck. Compatibility mode fails too.
And it was my first install ever. The rollback action occured at the very first time I tried to install : the setup begins flawlessly, and when it is about to finish, it stopped, then begin a roll back action and delete all that was installing, saying that "a problem occured".
I will try Calibre in Linux (Ubuntu) this week-end, but Calibre should have been more helpful in my windows PC.
Here's the installation process :
1. Installation begins, everything's fine

2. Copying new files, no problems

3. PAF ! Just as it was about to finish :

4. Damned stupid Windows window :