A tip for windows users: If you have recently purchased a copy of Windows 10 or have purchased a new computer that has Windows 10 installed, any key from win7 or win8 will work to upgrade Windows 10 to the same version.
Windows 10 Home, can be upgraded to win10 Pro using a key from win7 or win8 Pro.
Windows 10 Home, can be upgraded to win10 Enterprise using a key from win7 or win8 Enterprise.
Windows 10 Pro, can be upgraded to win10 Enterprise using a key from win7 or win8 Enterprise.
After testing the Windows 10 upgrade suggestion above, I advise anyone who wants to try this...don't. Microsoft does not allow this kind of upgrade to their operating system. I used my purchased Windows 7 Ultimate DVD key to upgrade my Windows 10 Home to Windows 10 Pro. It worked for about two weeks, then I noticed a watermark in the bottom right side of the screen notifying me that Windows wasn't activated. Tried the Windows 7 Ultimate key again and it was rejected. I figured out that the only way to activate Windows again was to reinstall the operating system. I went online and purchased an original Windows 10 Pro key and reinstalled the OS. Not really a big deal, all my app settings were saved online and I never keep anything important on the same drive where Windows is installed.