Won't boot all the way up

I am trying to fix this for a friend. It is a white macbook, I don't know for sure whether its an a1181, the writing on the bottom is rubbed off the case. When you hit the power button it will get to different stages of the start process, but it doesn't stop at the same spot consistently. sometimes I can log on and start toying with it, other times it will get to the apple screen and stop, and still other times there will be just nothing. any help will be appreciated, thanks