I just replaced my HD myself.
1. Unscrew the battery (4 screws) and pop it out.
2. Underneath, you can see the HD. It's in a case held in by two screws, look on the side closest to the middle of the laptop. Unscrew them.
3. Opposite the side with the screws are two plastic holders. Don't break these, these hold the sucker in, so remove the case from the side that you unscrewed from first.
4. Remove the data cable from the HD.
5. There are four screws on the outside of the case holding the HD in, unscrew these too.
6. Slide the HD out of its case.
7. Take your new hard drive and replace it, reversing your steps.
If you need more help or if these instructions aren't detailed enough, I can take pictures for you.