Thanks a lot for reply, Tech.
I don't think my computer was infected by virus, because I use avast over 4 years (I scheduled a Boot Time Scanning month ago).
I try to start Avast (I click on the blue ball icon on the desktop), but after checking memory and autostart (without any problems), the avast shutdown - it's crazy!
where is the problem? what should I do?
I'd be grateful for all your suggestions.