Looking at the back of the case. There are 2 screws along the left side of it. Just remove those 2 screws, and that side part of the case will slide towards the back. [br]
The Power Supply appears to be held in by 4 screws, and has various tie down points within the case for the cables. [br]
Ensure the machine is powered off, and disconnect the red/yellow/back cables leading from your power supply to the motherboard. These generally have notch style locks that you can easily undo by applying a bit of pressure, and pulling straight out.