Well ....
I noticed that Windows Defender is activated ! I don't understand, because I have my pc since 1 year, and I remembered I uninstalled it ... When I deactivate it, there is a second error message from the flag : windows defender is not active ....
So I don't think it is a problem with avast ...
And it is strange, when I type F8 to go to safe mode, there is not the usual screen, they ask me the windows version I have, so there is only 1 listed : windows 7. I didn't click on it, it is the first time I see it, so I feared to do something wrong. There is not the usual screen when we want to start in safe mode, that's weird !