If your computer is too old to upgrade to Windows 10 you could consider a Linux system like Ubuntu, Xbuntu, Lubuntu, Mint, Alpine, Puppy Linux, Tiny Core, LXLE, ARCH Linux. These systems based on UNIX much like Apple, Android.
These operating systems are free and don’t require antivirus software, come with free Office and email software. There are many different styles names and versions, you can try out a few different ones to find which one suits you better.
Windows alternatives for older PCs