Windows 8.1 and Windows 7 will get free upgrade to Windows 10 for the first year. So if you running 7 or 8.1 on anything, you will get a free upgrade to windows 10 when it comes out. More info to follow.
Source
Free Windows 10 upgrade 01-21-2015 - Slickdeals.net
Source
Free Windows 10 upgrade 01-21-2015 - Slickdeals.net