Insert USB into your old PC. Boot from USB (press F12, ESC, or DEL during startup). Install Windows. When asked for a key, click "I don't have a product key."
Select "Create installation media (USB flash drive, DVD, or ISO file)." Click Next.
Select "Windows 10 Home" or "Pro" (depending on your license). Click Next. Perform a "Custom: Install Windows only (advanced)." Delete all old partitions. Install.
