If you must use Windows 8.1 for legacy software, buy a cheap license key (they cost $5) and install Microsoft's official ISO. Then, immediately disable the network adapter or put it behind a firewall.
If you need a lightweight OS for old hardware, use or ChromeOS Flex . They are free, legal, and secure. ghost windows 8.1
There is a spooky corner of the internet where Windows 8.1 never really died. It’s called the “Ghost” scene. If you must use Windows 8
The Curious Case of Ghost Windows 8.1: Haunted OS or Just a Bad Install? ghost windows 8.1