DON’T Ever Reboot my computer without asking me!
I logged on this morning, the little green window popped up and said a new version was available, click here to install. So I did and proceeded to start checking and answering e-mail. I was in the middle of crafting a reply, when the computer rebooted without warning! My work was lost.
Don’t ever cause a reboot without asking the user if it’s okay!
It doesn’t on completion of the update if a reboot is required it notifies you and you have the choice to reboot then or not.
So I have no idea why you are saying it rebooted without asking the user. I have never had a program update that failed to ask for a reboot in over four and a half years of using avast.
Maybe you hit enter just as the little box came up asking to reboot? I know I have done that many times while typing.
avast never reboot automatically (in home version upgrades). You need to use Pro version with unattended installation. So, something went weird… maybe you click enter…
And as the other already said, avast ask you!
Avast always ask me, and I have updated many versions of avast, on my computer, and on family members and friends computers,
Just found out AVAST and ONLINE ARMOR FW FREE both upgraded at the same time. After reboot (in fact it didn’t finish rebooting…) I had a BSOD on VISTA SP1. The firwall acted like if it was the first time, I rebooted again and everything went smooth and OK. This last upgrade of AVAST brought me no problems at all. Thanks ALWIL!!! 
There are only two options that are possible and that can be changed.
Either avast! asks you if you want to reboot or it simply silently waits for you to reboot manually when you simply do that for some other reason. Or if you shut down the PC.