Cant adjust brightness

Hi,

I have XPS 15 (L501x)  laptop. I cant adjust the brightness of my screen through the function keys(quick set) or from Control panel (Control Panel\All Control Panel Items\Power Options)

I have already tried

• Reinstalling quick set
• Restoring
• Reinstall Drivers from my Drivers CD
  
  • Chipset : Intel HM57
  • Video drivers : Intel HD graphics

I also tried to install bios drives, but the system would allow me, saying, its already the latest

Please advise

thanks