Secure Boot will only boot Win8 so must be disabled.
From there you can install Win7 in UEFI (Unified Extensible Firmware Interface) - Install Windows 7 with - Windows 7 Forums
- or -
follow the steps I gave above to Bypass UEFI to Install WIn7
- this link tells more about Secure Boot and how to disable it and bypass UEFI to install Win7 normally to an MBR disk.
Here is more information including a video on UEFI so you understand it better:
The 30-year-long Reign of BIOS is Over: Why UEFI W... - Input Output
Technical Note: UEFI BIOS vs. Legacy BIOS - YouTube
You may want to try UEFI install first to see if you like its enhanced BIOS features, but if it becomes too much trouble it sometimes is easier to bypass it.