You will need a trial version of Windows 7. Unfortunately, all the Windows 7 versions have been pulled by Microsoft since Windows 10 has been released. You will have to obtain the ISO through another ways to install it, then use the serial number number on the back of the laptop to activate it when it asks. Unfortunately, this is the only way or you could buy a new copy of Windows 8.1 which is still available. A new serial number will be provided to you and a CD to install Windows, because Windows 7 has reached end of life for selling procedure.