How often has it happened that you have inadvertently hit the Caps Lock key and continued to type … ???
In Windows 7, you can set it hear a warning tone when you press the Caps Lock, Num Lock or Scroll Lock keys.
Open Control Panel > Ease of Access Center > Make the keyboard easier to use
Check the Turn on Toggle Keys checkbox.
Click Apply > OK.
Now you will hear a tone when you press Caps Lock, Num Lock or Scroll Lock.