If you want to install Windows 7 instead of Windows 8 you have to perform some changes in the UEFI firmware first.
- Enter the UEFI firmware (formerly known as BIOS) by long-press the F2 button and turn on the notebook.
Go to the "Security" tab and change the value of the "Secure Boot" option to "Disabled"
- Go to "Advanced" tab -> "System Configuration" -> and change the "Boot Mode" option from "UEFI Boot" to "CSM Boot"
- Save and close the UEFI settings by pressing the F10 key.
If this steps are done, Windows 7 can be installed on the Notebook without any problems.