Installation problem OS
I updated OS 2.7.4 to OS 2.8.3 on Win 7 and it could not start OS anymore. Then I updated Python from 2.7.2 to 2.7.8 andI reinstalled OS 2.8.3. Now everything works fine.
So on windows OS can, if you want to install new OS version, sometimes it may be important to update Python too if your Python version is outdated.