First download and install the latest 5.0.545 and see if that will fix it...
http://filehippo.com/download_avast_antivirus/These are the update settings that works for me.....
top right corner > settings > updates
ENGINE AND VIRUS DEFINITIONS
(ON) automatic updates
PROGRAM
(ON) automatic updates
UPDATE PARAMETERS
(OFF) i only connect to the internett using a dial up modem
(OFF) my computer is permanently connected to the internet
PROXY SETTINGS
(ON) auto detect (use internet explorer settings)
On some computers this have have worked
UPDATE PARAMETERS
(OFF) i only connect to the internett using a dial up modem
(ON) my computer is permanently connected to the internet
PROXY SETTINGS
(OFF) auto detect (use internet explorer settings)