* Teach them basic principles about scams and malware. Never let them install software by themselves.
* Explain what spam is and to not open emails they don't recognise.
Get them an iPad if they don't need a complete PC. This is absolutely the best option in my experience:
* Almost no maintenance required. Apps/OS upgrade automatically.
* Installing software is safe.
* Malware is basically non-existent.
* iOS UX is perfect for basic users and interface is usually self-explanatory.
If they absolutely need a PC:
* Teach them the basic tasks they want to perform. Searching the internet, online banking, photo management etc.
* Install a modern auto-updating browser, e.g. Firefox.
* Install an ad-blocker, e.g. Ublock Origin. This gets rid of scammy and confusing ads which may install malware.
* Install a remote assistance tool, e.g. TeamViewer.
* Enable automatic OS updates.
* Enable firewall.
* Depending on the OS, install some sort of basic anti-malware software.
* Uninstall or remove/hide shortcuts to applications they won't need.