If you don't have enough space (a free partition) for Windows 7 then shrink an existing partition to create at least 30 GB free space.
Install Windows 7 normally using DVD/USB onto the free space - Windows 7 will create a NTFS partition by default.
Download Visual BCD Editor from http://www.boyans.net.
It comes with "Dual-boot Repair" utility.
Run Dual-boot Repair, click on "Automatic Repair" - this will set Windows 8 back in control of the booting. Windows 7 will be a dual-boot choice in the menu.