The OP seems to know that different users can have their own windows accounts, and the difference between an administrator and a limited account. This makes them more computer-literate than about 80% of Windows users IMO, and well able to handle a modern Linux system. Linux is not hard to use, just different. An old XP or Vista-age machine would get a new lease of life - and be far more secure. But you need a lightweight version of Linux.

BUT they would need help to install and familiarise with it and that's hard to find unless a relative uses it. Some UK Linux User Groups put on events at the time of XP's demise where you could try it out and get a local mentor.

And, of course, it depends what you want to run on it: it's not a good choice if you need Windows-only applications.