The cheapest way to replace Windows is to use a free operating system such as Linux. Linux is an open-source operating system that is similar to Windows but is free to use. There are many different versions of Linux, so you can find one that fits your needs. Linux is not as user-friendly as Windows, so it may take some time to learn how to use it. However, there are many online tutorials and forums that can help you get started. Additionally, there are many companies that offer support for Linux, so you can get help if needed.