it works for me now after downloading the main installer again. The installer asks to update the program and after a system restart, the virus database update works without errors. http://www.avast.com/i_idt_1016.html is where you can get avast 4 Home
There were problems with ‘date and time’ on avast! servers last week. Vlk posted this answer in related forums. If everything is working now, let finish the thread ;D