Latitude C400 orange light flashing ominously...

I've got a Latitude C400 laptop that has served me well for a couple years now. Recently had to reinstall Windows XP when it caught a little virus but seems fine now. Only one problem: the orange flashing light. Just below the monitor are the three little lights to indicate power on, hard disk activity, and battery charge. Usually all three are a happy and contented green unless the battery is almost dead. But since Windows was reinstalled (total coincidence? probably) the indicator light with the picture of the battery on it has begun to flash most insistently from the second I turn on the computer until I power down. In the past it would flash every now and then when it got a little hot. But now it's flashing 24-7, regardless of whether the casing is warming up or not.

It seems to be genuinely worried about something but I honestly don't know what it's trying to tell me... Help!