laptop won`t power up?

hi i have an old compaq presario 700 laptop that has never powered up it charges but when you switch it on just the green light comes on and a very quick but faint noise as if it wants to start . I have changed the motherboard and put the original processor in ( as the newer working board didn`t have one )put it all back together and the same happens anyone have an idea what it could be or i`m i just wasting my time trying to fix it