http://support.hp.com/us-en/document/bph07107



On some computers, the motherboard can flash or
change the color of lights on the outside of the case, such as the power
button LED and the hard drive activity LED. For computers that support
this technology, lights blink and provide a beep to indicate a failure
condition.

Example of LED conditions: click a menu item to view the condition

LED and beep conditionDescription
Power button LED blinks 4 times with 4 beepsPower Supply
Power button LED blinks 5 times with 5 beeps
Memory
Power button LED blinks 6 times with 6 beepsVideo Card or Graphics
Power button LED blinks 7 times with 7 beepsMotherboard or System Board failure
Power button LED blinks 8 times with 8 beepsBIOS or CMOS problem (ROM)