Once you install windows 8 on a system it will not allow you to install a prior OS like windows 7, this is because of the way the file system is layed down on the HD. Windows 7 uses a NTFS file structure where as Windows 8 Uses a GPT file structure. The data is layed down and located on different parts of the drive. That is why windows 7 cannot install due to the Gpt format of the drive. As it is an older operating system and does not understand GPT format.

 

Boot from USB/DvD

On setup screen press

“SHIFT + F10″ OR” SHIFT + FN + F10

listdisk
DISKPART
Select disk 0
Clean (will erase all disk)

Close cmd
Install win 7

 

* Problem : While Installing windows 10 from usb

Error Message :   a media driver your computer needs is missing , while installing window 10 from usb

Solution :

• Download last iso win 10 ( by microsoft media creator tool)
• Use the same procedure to install windows 10  ( with disk park )