follow the steps and pictures in the HDD replacement thread to gain access to the internals, which includes removing the keyboard and base screws.
After that you will need to remove DVD drive, HDD and various other internal parts.
Finally you will need to remove the motherboard, as this is necesssary unbolt the fan.
My advice to you (after changing my own mobo) is to take some pictures of the internal components before you start removal of internal bits.