The program that runs the On Screen Keyboard is "C:\Program Files\Common Files\microsoft shared\ink\TabTip.exe"
There is also the original On Screen Keyboard from earlier operating systems still floating around in Windows 8.
C:\Windows\System32\OSK.exe.
Maybe your Keyboard is damaged and the Key needs replacing.
How is your computer in Safe Mode? What is it like if you test with another user?
How to start Windows 8 in Safe Mode
http://www.bleepingcomputer.com/tutorials/start-windows-8-in-safe-mode/
Create a user account Windows 8.1
http://windows.microsoft.com/en-au/windows/create-user-account#create-user-account=windows-8