How Secure Boot works
Secure Boot Configuration is a new feature in BIOS 8 that allows your computer to validate the authenticity of components like hardware and firmware. When your computer was manufactured, UEFI created a list of keys that identify trusted hardware, firmware, and OS loader code. It also created a list of keys to identify known malware. When Secure Boot is enabled, the computer checks that everything being loaded and used even before Windows opens is safe and recognized. Secure Boot also checks the validity of critical hardware, including the motherboard and video card. Any malware or other firmware code that is not recognized is blocked. For example, Secure Boot can prevent your computer from starting from illegally copied CDs or DVDs that could harm the computer. Secure Boot does not lock out valid recovery discs or Windows discs.
If an older model video card is installed in the computer, Secure Boot might not recognize it as valid hardware. If the card is not recognized, Windows does not use the card when it boots up, and you may experience problems starting the computer, or there might be no video output at all.
To resolve the issue, you can use a video card that Secure Boot recognizes, or you can disable Secure Boot and enable Legacy Boot in the BIOS. This document explains how to disable Secure Boot and enable Legacy Boot.