but it takes forever to start (avast is inacitive for a minute after desktop appears
Let me see if I can explain this in easy words.
Thinking, thinking ouch that hurts

(just kidding)
When you start Windows (boot up the system) many things are loaded/started. Windows however does not have a way (by default) to let you choose what is loaded 1st, 2nd and so on. It seems to load the things randomly. Windows also doesn't wait for a application to be started/loaded fully at boot time before it loads the next.
Somehow (I don't know the exact technical details) when you have a softeware firewall installed as well as a av application (not only with Avast and ZA) one is waiting for the other to be fully loaded/started before it starts doing its job.
This is likely not a problem with how the firewall or av works, but more like how windows is working.
If I recall correctly there is a application mentioned in one of the threads on this board which will let you change the way and order in which applications are loaded during boot time.
Just a suggestion:
A router with build in hardware firewall is about 40 euro (+/- 40 us dollar) and offers more security than a software firewall. This is about the same amount of money you will have to pay for decent software firewall.
I hope this info will help you.