A Pre-installed version of Windows 8 will normally be done using the UEFI configuration. The way your describe your partitions, it sounds like yours is using Legacy (MBR) configuration. Do you show an EFI partition in Disk Management? If you have a "Windows Boot Manager" it also indicates the install is UEFI.
But Windows 7 will be recognized either way. If Windows 8 is set for secure boot, you will not be able to boot to and install Windows 7 until it has been changed to enable the CSM (Compatibility Support Module). This is done differently on different systems, so I can't really tell you how. After you change the secure boot, you will have to boot the Windows 7 install DVD in UEFI mode by selecting that version using the Boot...