I have an old Windows machine, that's dying from how terrible Windows is. I'd like to switch to Gentoo, but to avoid many problems I'd like not to dual boot. I'd still like to keep my license to Windows though, because, y'know, it costs $100+.
Therefore, is it possible to uninstall Windows, keep the license, and install it on another machine later in time if needed?
You need to consider what operating system is on the machine.
If Windows 10, the license for Windows 10 is maintained in the Windows licensing server and you can always install Windows 10 on the same machine.
If the machine is Windows 7, you need the installation media and key to reinstall. Microsoft does not keep the Windows 7 key.
If the machine is Windows 8, you need the installation media but the system has the key and you can reinstall Windows 8.