I am sorry to hear that have had issues with function keys on your computer.
Please reply with more information about your computer configuration, I will need the exact processor model and the operating system version, if 32 or 64 bit.
Use the search in Windows 8 (swipe from the right side of the screen) and type "System"
Under System Type you'll see either 32-bit or 64-bit and also the processor model.
Please bear in mind this type of function key configuration is customized by your computer manufacturer and by loading generic Intel drivers, the keys will stop working; therefore, your system will need customized drivers.
Thank you for the information.
Please try updating to the latest video drivers available on the Intel site:
As I mentioned, if the problem remains, you will need to get customized drivers from your computer manufacturer or motherboard maker.
Additionally, you can change brightness settings through the graphics driver control panel or using windows power options.