Couple of things to check: Do you actually have the latest database?
-Right clikc Avast icon in the system tray, select "about", look at the date on the VPS file.
Is your system time and date set correctly?
It might be necessary to reset the repository. If the above checks don't fix this, please post your operating system. (Post the VPS date and version too, please, and confirm the Avast version is 4.8.1335.)