Thank you for your very quick answers!
In fact, the only antivirus I had before Avast was Trend Microsoft Internet Security and I have uninstalled it before installing Avast. But I am almost sure that I never have installed MSE (Microsoft Essential Tools). Is it normal to find an update for MSE on my computer?
Let me just give you one more information. In fact, I performed a complete scan today because this afternoon I had two alerts for a Trojan
Avast said it was blocked, but I wanted to be sure there were no problem by performing a complete scan.
The result was that there are no file infected, but that some files cannot be scanned.
If you say that there is no problem, I think I will simply do nothing since there is a high probability that my computer is not infected.
Thank you very much for your help!