Ask HN: How do you protect your older parents' PCs?

My parents are both not that great with computers. My father can have trouble distinguishing between an ad designed to look like a Windows alert, and an actual Windows alert. My mother has woken me up out of bed to tell me she'd followed my instructions for opening a website but it "didn't work", to find that she had typed the URL in to the browser bar, but hadn't pressed "enter" yet :). But luckily their usage patterns are simple - they browse websites (most the photos they look at are on Facebook now), occasionally will need to read a MS Word doc one of their friends sends them, or sometimes open photos from emails.

Also its a shared PC that visiting relatives, kids and teenagers sometimes use - none of who are particularly computer savvy, and are generally motivated by games. I've found games and things on it that my parents would clearly have zero interest in (most likely the source of the adware).

I recently used their PC and was also shocked to see that Chrome's default search engine was some crapware. There was also some clearly non-Lenovo bloatware installed to "keep drivers up to date" which I spent some time obliterating. Short of MS Security Essentials and locking down the admin account, I'm looking for additional suggestions.

I'm sure a lot of us here can relate, and I'm curious as to your thoughts, tips and techniques. The internet is simply full of paid product offerings that claim to do it all for you, but I'm massively skeptical of these products.

The limitations: I actually get to their computer maybe once or twice a month to do a health check. It's unfortunately gotta be Windows. Obviously not on my network.