ReactOS is a free and open-source operating system with Windows compatibility up to NT 4. See more at www.reactos.org.
These are the latest unmodified Windows 10 ISOs mirrored from Microsoft. If you already have Windows 10 installed, you can use Windows 10 Update Assistant to update to the latest version.
You can buy cheap activation keys here.
These are the last Windows 7 ISOs released. You can install updates released after these ISOs with Windows 7 SP1 Convenience Rollup.