Linux is definitely the best solution, I recommend this distribution made specially for low resource computers.
It looks like windows xp but work great for old computers ! and security is no more a problem when updated.
You can test it in live sessions before installing it too so you can see if you will like it
Make sure to backup your photo, music, video, bookmarks and everything you want to keep on usb key/external hard drive before installing it.
download at :
https://www.lxle.net/
there is a lot of tutorials how to install too :
https://www.linuxhelp.com/how-to-install-lxle-16-04-1
