Try a repair of avast. Add Remove programs, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
I think you have found the problem. In fact I installed in C\Sécurité. I have others things such as my firewall installed here, so I think I would better try the reinstall
avast can’t be installed in folders with non-ascii (latin) characters, like the two accented é in your case.
It’s a pity that the setup does not warn the user about that.