The Windows 10 is supposed to be for computers which already have a licensed copy of Windows 7 or 8. For a freshly built computer with no OS they are going to want that copy of Windows registered. Do not be surprised if they want a product key added and activated shortly. The free upgrade is per computer not just someone who has another computer with a legit copy of Windows. Don't be surprised, just saying.... and none of the !@#$%^ Microsoft !@#$%^ if they do because honestly, operating system cost something... It was pretty amazing that they gave current customers free upgrades....
By the way, if this does happen it is pretty simple to install Windows 7 and then do the upgrade. During the upgrade you can do a full clean install of Windows 10 anyway. The windows install really is tied to a computer. I totally understand any company protecting themselves... If you are building a new computer from scratch, one of the components that you also need to purchase is a copy of Windows. You cannot use one copy on multiple computers.... again, just saying.... (i.e. I have five computers which had five paid for copies on Windows 8.1, and now all five are running Windows 10).