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
Select disk 0
Clean (will erase all disk)
Install win 7