Incorrect Connections

• Some models of keyboard will flash if they are connected to the wrong port on the PC's motherboard. Double-check that the keyboard cable is plugged into the right port and is firmly connected.
  
  

Low Battery

• If you're using a wireless keyboard, flashing lights are often used to indicate low battery power, in which case they will need to be swapped with fresh ones. Replace the batteries in the unit to see if the problem is fixed.
  
  

Hardware Fault

• Flashing lights can also be used to indicate a fault with the keyboard itself -- consult the Support section of the website of the hardware manufacturer and the Knowledge Base of the Microsoft website. Use the exact make and model number when searching. You can also try downloading the latest firmware updates for the hardware from the manufacturer's website.