Click on the ‘a’ blue icon on system tray.
Choose the provider that you want to start (resident protection) at left.
Click on Start and check to ‘persist’ the changes for the future.

If you do not ‘have’ the ‘a’ blue icon on system tray, see http://forum.avast.com/index.php?board=2;action=display;threadid=3542;start=0