Yes, you could go Linux. My grandma loves it. Easy to use firefox for browsing, do a bit of emailing and similar. Very fast, efficient, and free of nastiness.
I like Deepin, it looks great and has a fab media player/converter and excellent software centre if you like that sort of thing.
Linux Deepin - Browse Files at SourceForge.net
Now has Kent university mirrors so updates are a breeze.
Or , if you prefer proprietary lock in os , you can buy win8.1 at amazon, it is relatively cheap as it isn't very popular.