Powerbook Problem

A friend of mine has a powerbook G4 (don't know the specs) that recently stopped booting. She bought disk warrior, and that didn't seem to fix anything. She got all the data off her hard drive and onto her new macbook pro, but she still wants to fix her old powerbook. It will turn on and the display seems to work, it just won't boot into an OS (i think an error message shows up or something like that). I assume that it is a hard drive failure, but can anyone confirm this? She seems to think that it is either the hard drive or the logic board, but from what i know about logic boards (which is very little) her problem doesn't sound like it is related to the logic board.
Also, how hard would it be to replace the hard drive (assuming that is the problem)? I have never replaced a laptop hard drive and neither has she, so would it make more sense to try to get an apple tech to install a hard drive?