After a bit of experimentation, I've found that I still cannot run the Avast! firewall on my desktop, as it seems to block inbound network connections for some reason. I've checked & double checked all the settings and there shouldn't be a problem. I had to revert back to Antivirus only again, & use Windows Firewall.
Seeing as how I rarely use the desktop to access the internet, this is no big deal to me, as my laptops running AIS now seem to access the network normally & streaming also seems OK. I would imagine that this might be a problem for other people though, & at some point I hope I can go back to AIS on my desktop also.