Windows OS: How to get it?

[icantbiuldpcs]

For my PC that I am building, i was looking up how im gonna get windows operating system on my PC. Its looking like im gonna have to buy windows 10 which is 100$. I already have windows 10 on my personal laptop (The one im building is for my son) But will i still have to pay for that or can i somehow get that windows cd key thing that i think i need? Thanks!

 

[Gam3r01]

You can use a license on an existing system, but it can only be active on one system at a time. So thats a no go.
There are two valid options, buy retail win10 for 100 bucks.
Or
Use it unactivated:
Totally free and legal, full spec win10 minus personalization (no wallpapers), but free forever:

https://www.howtogeek.com/244678/you-dont-need-a-product-key-to-install-and-use-windows-10/