I uninstalled 2.50.0 from the control panel and tried to install back 2.49.0 using Windows 32-bits installer from:
http://download.calibre-ebook.com/2.49.0/
I again had to save it to my computer and run it from there to skip the unsafe notice.
The process seems to complete but I cannot get the program to run even after a repair run from the control panel.
Suggestion ?