It wasn't hugely advertised, but Win 7 had something called "XP Mode" where it would literally start windows XP in a VM completely in the background, and any program you wanted to start would actually run inside that VM, with windows drawing the application window like a native Win 7 app. It was magical in how well that worked. And yeah, MS just provided a special XP image for this, you didn't need a separate licence for it.