If you are building your first PC, you will have to buy a new copy of Windows 10, OR use an older legitimate copy of Windows 7 or 8.1, then upgrade for free to Windows 10. The idea behind the free upgrade was to encourage existing users to move to Windows 10. New users will still have to purchase the OS on their own. Note that if you have the option to buy a legitimate copy of Windows 7 or 8.1 for cheaper than a brand-new Windows 10 installer, you should take this option. Install 7 or 8.1, then upgrade to 10 for free.