Why not try Linux? Created in 1991 by Linus Torvalds Linux is the free alternative to Microsoft Windows and Apple MacOS. Linux can (in most cases) run on either Mac or PC, there are even some distributions for phone and tablet. Linux is Open Source which means it's free and the source code is open for developers change at will.
My particular favourite distribution of Linux is Ubuntu (roughly translated as "Humanity to others"). Ubuntu is a great way to get the most out of an old PC or Mac without having to pay out for an operating system and office software, Ubuntu can be downloaded free of charge from https://www.ubuntu.com/download/desktop. When it comes to Linux there really is such a thing as a free lunch!
If you need help installing or learning to use Ubuntu call The Computer Wiz on 01553 660941 or email email@example.com.