Things to check:
-1) Are you running from an admin account?
-2) Apart from the out of date database, is Avast working?
if so,
-Right click the icon in the system tray,
-mouse to "Updating" then left click on "program update"
The program should then spend a little time downloading the latest version. Or a longer time if you are on dial up.
Once it has updated, it will probably want a restart.
Then check for VPS updates. If you use a third party firewall, you will need to allow Avast again, as the program has changed.
Any problems at any step, post the problem and any error message.