Th problem now is my father has asked for Windows to be installed (dual-boot)... he thinks it's "more compatible".
It's not really an ideological problem. It's their choice. But it would be a pain in maintenace, as they don't know how to do updates and use anti-virus, firewall, etc...
Windows can be set up to get Windows Updates automatically, Same for antivirus. Most AV programs get updates automatically. Are your parents behind a router? NAT in the router should be sufficient protection. I don't recommend software firewalls for non-technical users. Queries about whether to allow outbound connections are very confusing and most people aren't sure what to say. If they say no, they risk blocking things they need. If they routinely say yes, what's the point of the outbound monitoring? It just wastes their time and slows down their system.
I don't use automatic updates (except for AV) on my Windows systems, but for most users I advise, I do have them use automatic updates because they don't remember to do manual updates and it's worse to be unprotected.