You haven't told what version of avast you are using.
I assume it is the free version.
Without knowing more details, I have to guess.
If you are using the free version, I suspect that avast installed the latest version to solve the automatic installation of the SafeZone browser.
Let's see if I can keep it simple
There are multiple types of drivers.
Most people know drivers as software that make it possible for hardware (e.g. printers, scanners, mouse, keyboard) to communicate/be used with the OS.
The driver in question is a kernel mode driver.
Without kernel mode drivers no OS will work.
The kb970158 hotfix solves a security flaw in the OS.
avast and other security software are using (ofcourse) parts of the OS to function.
If there is a security issue with the kernel driver, the security software is also at risk.
That is the reason why avast installs it.
If you want to secure your house, you can put locks (=avast) on the front door.
But it will be useless if you leave the back door (=Windows) open.
This (and other hotfixes) are making the back door more secure.