1. If your laptop comes with Core 4th generation (ix-4xxx), the windows key already wrote in the BIOS, just download W10 and make a USB! Use the USB for a clean installation of your windows. When it ask you for the key, just choose I have no key, it would activate once the installation finished.
2. If your laptop is a very old model, when it ask for the key, input the W7 or W8 key on the sticker (COA) would be works !