Using Avast 7.0.1407 and Windows 7 Pro x64. The tray icon is intermittently is deactivated although Avast is still running. It is reactivated when running the program through the start menu shortcut.
You can try a repair of avast first via programs and features and reboot when finished and see if that fixes it, if that doesnt solve it there is a fix tool that was released to fix some issues with a corrupted installer in the earlyer released versions of 7.0.1407 http://public.avast.com/supp/FixUpdate.exe