You can try this :
Instructions for manually disabling secure boot:
1) Power down the system
2) Remove the NVIDIA Add-in card
3) Boot the system using integrated graphics
4) Enter CMOS settings. CMOS settings can usually be accessed during boot, typically by pressing one of F1, F2, F8,
F12, or Delete (depends on the system firmware) Alternatively they can be accessed in Windows 8 as follows: http://www.makeuseof.com/tag/how-to-access-the-bios-on-...
5) Set Secure Boot to disabled
6) If there is an option, set CSM (or compatibility or legacy mode) to enabled.
7) Save the new settings
8) Power down the system
9) Install the NVIDIA Add-in card
10) The system should now boot then install the graphics driver.
* Note that not all existing NVIDIA based cards are capable of adding UEFI support. If your card manufacturer is unable
to supply you with an update, you will need to operate in compatibility mode" (nvidia.com)