A little while ago, I was having problems using installing Arduino drivers on Windows 8. Turns out, you can’t install unsigned drivers on Windows 8, without jumping through some hoops.
I searched around and found this post on DIY Drones.
Long story short, open a new Command Line with Administrator Privileges, and issue the following commands:
bcdedit -set loadoptions DISABLE_INTEGRITY_CHECKS bcdedit -set TESTSIGNING ON
After that you can reboot, and you’ll be in “Test Mode” with driver integrity checks disabled ready to install your Arduino driver (or any other unsigned driver).