You have lost you CD/DVD of Windows 10 Professional N 64 Bit (x64) or you have bought a computer but you don't have the CD/DVD of Windows Seven ? It is not a problem, download a .iso for free. With this .iso of Windows 10 Professional N 64 Bit (x64) you can install your computer for free, but you need to have a license to activate Windows 7. You can found your license under your laptop or on the case of your computer.
Tutorial of the installation : How to install Windows 10