Windows 10 (to which I was introduced today) won't allow me to[b] enable Windows Defender [/b]because it [b]believes Avast already has a firewall in place[/b]. This is despite the fact that I have uninstalled the Avast firewall component on the understanding that it would allow me to enable Windows Defender.
Windows defender and windows firewall is not the same thing

https://support.microsoft.com/en-us/help/4028544/windows-turn-windows-firewall-on-or-off