Well you could start with the missing information, sorry couldn't help my self
What are your settings in the avast Program Settings (right click the avast 'a' icon), Update (Basic) for the Program and Virus Database ?
See image for the default settings.
Are you getting any errors or warnings displayed ?
Check the avast! Log Viewer (right click the avast 'a' icon or C:\Program Files\Alwil Software\Avast4\ashLogV.exe), Warning or Error section.