Motherboards usually come with a DVD containing the software *drivers* for the hardware, NOT a copy of Windows.
As pointed out, you can download and install Windows 10 and run it unactivated indefinitely. A few personalisation items will be greyed-out, but you can even mostly circumvent those. Everything otherwise will run perfectly fine.
You can also buy an OEM version of Windows 10 for that specific computer, at somewhat lower price than a full version. Then you can "legitimately" activate and so on. It's not transferable, but it's probably enough; else-wise, buy the full retail version which is transferable to some future computer.
For so many reasons, do not pirate or buy at a super low price from a shady website. It's a form of...