Short answer is, you can't. Avast has a self-defense service built-in which prevents malware from turning the service off. If you want to disable YOUR version of Avast, then you can right-click the Avast icon in your task manager and then navigate to Program Settings > Troubleshoot > Disable Avast self defense module. From there, you can then remove Avast from the msconfig to disable Avast from auto-starting with the system, then reboot into an Avast free setup.
To be fair, if you would learn to program device drivers, you could subvert the self-defense service and disable it that way. But that takes us more in the direction of malware... and you've been warned about that already.
HtH
