This Dell Support, and the Service Manual for the Dimension 8400,http://support.dell.com/support/edocs/systems/dim8400/SM/index.htm
In the Service Manual main menu, left-click on - Opening the Computer Cover
1) Computer off, and unplugged from power. (Always unplug the power cable from the surge protector. Then if the power cable needs to be removed from the computer, do so)
2) Remove all cables to the computer
3) Lay the computer on it's Right side.
(As you face the computer, your right side is the computers Right)
4) There is a Release button on the Top of the computer case, and a Release Button on the Bottom of the computer case.
These are located at the Rear of the computer.
The preferred method of opening the computer case, is to press one release button in at a time, and slightly lift up on the side of the computer case, where that release button is located near.
Then go to the other side of the computer case, press this release button in, and lift up on the computer case.
The computer case opens like a clamshell. The computer case is hinged at the front.
Computer laying on it's right side. Press the Top release button in, and lift up on the computer cover located near the Top.
Now press the release button on the Bottom. Lift the computer case up.
The computer case opens from the back, towards the front, like opening a car hood.