Hello All!
I believe I found out what is happening in windows 8....
When you shut down windows 8, in fact it enters in hibernate state. Only restart makes it really shut down the computer before it turn on again.
So, when you turn on your windows 8, it will make its start from the latest state of you machine.
In other words, lets say avast updates right now. And you turn off the computer right after the update.
Two days later you turn on the computer again. It will not auto update as soon as you turn on the machine (like happens with windows 7, vista, xp, etc). You will have to wait 4 hours with the computer turned on, to avast starts its auto update cicle (default is 4 hours) and you be able to get the latest definitions...
Thanks for your time!
Elminster