It happened to me too. And when I tried to start the service manually using Component Services, I got Error 1053
http://i.imgur.com/i7snl34.png

I have Avast! Free 2014.9.0.227
Windows 7 Ultimate 64 Bit Service Pack 1
Smadav 2013 Rev 9.4
Avast! Free Version 8

Previously I tried to clear uninstall Avast with the tool I’ve got from http://files.avast.com/iavs9x/avastclear.exe
And then after reboot, I installed Avast again. It worked for some times. But after a several days of use and several PC reboots, it happened again.
Sometimes after reboot, Avast worked again normally, and then after another reboot, it wont work.
I noticed if I reboot, Avast icon is late to show up on the notification bar compared to other programs (ex: cFosSpeed, Dropbox, XFastUSB, IPMSg, etc)

How can I get Avast to work again? Thanks for the help