I've seen many resident nasties that are in fact created and installed on your system by an anti-virus vendor. These typically are not legitimate anti-virus products at all and tend to be viruses/trojans themselves. FREE ONLINE VIRUS SCAN often equals FREE VIRUS INFECTION (pay to remove).
And more than one legitimate company uses sales tactics that approach this. I shouldn't name names (**cough** Norton **cough**)
So, I fired up the Windows machine, and Avast loads when it is supposed to very early in the boot process. (I don't use wireless, so I can't check that, but it is initiated before my network comes online, which suggests it would do that regardless. Now, the resident protection icon in the system tray doesn't come up immediately, but that's different than the underlying system. That is, for all intents and purposes, the UI, not the program itself. It may seem to come up late.
How are you determining that your wireless is coming online before Avast?
FWIW, you can schedule a boot-time scan that will run before the operating system gets loaded by going to the Options menu, opening Schedule Boot-Time Scan, then just clicking the appropriate buttons. Note that this takes a LONG FREAKIN' TIME, and actions will be automated based on your choices.
I pulled up the user manual (I never read them either) and found this bit in BOLD:
Note : various problems can arise as a result of installing more than one security
product on the same computer. If you have installed other security software, it is
recommended that this is uninstalled before you try to install avast!
Note that shutting down the UI for an anti-virus product seldom shuts down the anti-virus software as a whole. There are several different processes running with security software, only one or two of which will be the UI.
So, I'm not sure what caused your issue, but my suspicion is still the presence of another virus scanner getting in a fight with Avast when you installed it.