How to make bootable PenDrive for Windows 7/8 installation from Command Prompt

If you do not have a DVD reader in your PC, then how will you install Windows 7 or 8 in your PC..???

or

If you have that but don't have a blank DVD to burn for windows setup, then how will you install Windows..???

or

Your installation DVD has so many scratches on it, then how will you install..???