Since external keyboard is working fine, it looks like there's no problem in system software as well as registry. I think problem lies with the keyboard cirucuitry. If you have warranty left on your laptop then you can call acer's customer service and explain your problem they are obliged to change your keyboard for free. Few months back, I had a problem with my hp laptop keyboard. Two of the keys were popped out then I called HP's customer service and got it replaced. They even sent me packaging material and everything was free and it took only 5 days altogether (they sent me packaging material with return stamp, I sent them my laptop and they repaired it and sent me back).