Power up problem. My daughter's laptop is a Compaq Presario 300. When you press the power button,

Question

My daughter's laptop is a Compaq Presario 300. When you press the power button, the fan comes on briefly, the power lights flash amber for 5 seconds then go out. The hard disk does not spin. I've tried all the different combinations of battery and power supply and held the power button down for 1 minute. The power supply is new and working (checked with volt meter). The screen does not light up. I checked all connections and they are good. If I know what component to replace I can get buy it and do it myself as I have fixed laptops before. I'm sure this is not an unusual problem for this model. Thanks for any help.