cannot change brightness in windows 8.1

Suddenly my brightness controls stopped working suddenly after a reboot, whereby changing the brightnes using the hotkeys or in the setting only moves the brightness indicator but doesn't acctually change the brightness.

Here is what I tried but didn't work:

1. Adding EnableBrightnessControl in the registery
2. Disabling adaptive (automatic) brightness
3. Repairing boot camp drivers through the USB disk I installed them at
4. Reinstalling windows using the same USB in boot camp

After the troubleshooting I'm guessing that the drivers in the USB disk are corrupted, however I don't know how to reinstall them again without reinstalling windows along-which I don't plan in doing agian.

I'm using Boot Camp 5.1.2