0426-1 is current virus definition version rather than the program version.

Which program version do you have? right click the blue ‘a’ ball in the system tray then click ‘about avast!’

Will a repair help? Start>settings>control panel>add/remove programs then click avast! Antivirus and in the new window click repair>next

Also, when do you receive this error? At bootup or when a connection to the net is made?

Delta.