Hello Serge and WELCOME to avast! forums !!!
First of all, please post your configuration as detailed as you can (your OS, Service Packs and similar things). List of security programs would be great as well. We can help you only if you help us.
Do you maybe have any other antivirus program installed on your system in the same time ? Having two antivirus programs with resident shields running on the same computer, could 'cause huge problems... same thing with firewalls.
Cheers !