Generally, those three times, this is what happened:
1. Windows initially have some problems at start up: taking too much in loading all the boot components (just dropbox, intel drivers and the antivirus), taking too much in start programs, freezing with the simplest actions like opening a directory or a new tab in firefox. (and this for one and half day)
2. this morning, the pc stopped working. It reaches the point with the black background and the Windows blinking logo, and stopped there, for hours.