I'm assuming you're not asking on how to get it for free, but instead what are the requirements of previous versions of Windows?
With the full version, you are purchasing a completely new license for Windows 7. You are not required to have a previous version of XP or Vista installed nor activated. You can still do an upgrade from Vista if you want, but the full version doesn't have any of the requirements of having to have a previous copy of Windows like the upgrade version does.
I would not recommend downloading Windows 7 from any other site than the Official Microsoft Sites.
Additionally, it's always best to make sure you have a genuine version of Windows 7, this will entitle you to recieve updates and support from Microsoft with Genuine Advantage.
"Microsoft Genuine Advantage programs, including Windows Genuine Advantage, help you determine whether or not your copy of Windows is genuine. Genuine Windows software is published by Microsoft, properly licensed, and supported by Microsoft or an authorized partner, giving you full capabilities, access to all the latest updates, and confidence that you are getting the experience you expect. Microsoft continues to invest in education, engineering, and enforcement in order to more effectively combat software piracy. It’s part of Microsoft’s commitment to help protect its intellectual property and to help you avoid problems before they happen."