At first, thanks for help.

Well,

The application must keep active in the background (with the blue ball at system tray).
that's what I thought. But there's no blue ball and no avast active in the background. That's why I was so suprised.

I haven’t tried to repair it, I just reinstalled it. But I will try this and tell if it helped :wink:
The OS is Windows XP with SP1.

Thanks so far.