To boot Windows 7, you need to disable the Win8 OS optimized default settings, please follow the below steps.
1. Restart the PC and press F1 to enter Setup
2. Navigate to the Restart menu
3. Select OS Optimized Defaults, and change the value to Disabled
4. Select Yes to continue to disable the OS Optimized Defaults
5. Press F9 to load Setup Defaults
6. Select Yes to confirm loading default configuration
7. Press F10 to save and exit Setup
Now you can boot to the Win7 installer DVD...