How to Disable Driver Signature Enforcement in Windows 8:

From the Metro Start Screen, open Settings (move your mouse to the bottom-right-corner of the screen and wait 
for the pop-out bar to appear, then click the Gear icon).

Click ‘More PC Settings’.
Click ‘General’.
Scroll down, and click ‘Restart now’ under ‘Advanced startup’.
Wait a bit.
Click ‘Troubleshoot’.
Click ‘Advanced Options’
Click ‘Windows Startup Settings’
Click Restart.

When your computer restarts, select ‘Disable driver signature enforcement‘ from the list. You can now load your modified driver. 
Reboot again once the driver is installed and all will be well.

Source(http://forum.xda-developers.com/showthread.php?t=1809195)
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How to Disable Driver Signature Enforcement in Windows 8.1

From the Metro Start Screen, open Settings (move your mouse to the bottom-right-corner of the screen and wait 
for the pop-out bar to appear, then click the Gear icon).

Click 'Change PC setting'
Click 'Update/Restore'
Click 'Restart Now' under 'Advanced startup' wait a bit
Click 'Troubleshoot'
Click 'Windows Startup Settings'
Click 'Restart'

When your computer restarts, select ‘Disable driver signature enforcement‘ from the list. You can now load your modified driver. 
Reboot again once the driver is installed and all will be well.

Source: Tom (TuneECU Team)