As the title is asking, could you just use Windows 10 from another computers HDD?
If it's a prebuilt computer that comes with Windows, that copy is probably tied to that motherboard. If you purchase a full retail version of Win10, you could transfer it from one system to another, but not an OEM version.
So I couldn't get the key from buying a prebuilt?
I'd contact the manufacturer to be sure, but I would guess that Windows would already be activated on that machine, with an OEM key.