The On/Off switch is the Power On switch.
The switch itself is tiny, and is located inside the plastic Power On button assembly.
There are plastic tangs on the Power On button, which hold the switch in place.
I use a hair dryer set on low heat, to gently heat the tangs so they will spread easily, to remove the switch. (Move the hair dryer slowly back, and forth)
You can also set the Power On assembly in a bowl of Very warm water, to temporarily soften the tangs.
(The switch is no good anyway. Dry the plastic Power on assembly before installing the new switch)
This switch is usually a replacement, http://www.directron.com/atxswitch.html
To remove the Power On button assembly the computer case is opened.
BEFORE you start, the computer Must be unplugged from power, and you should observe anti-static precautions.
Your body carries Static electricity. Static will fry out (Short Circuit) the components inside a computer.
You can either,
1) Buy a $6 ESD wrist strap, connect the wrist band to your wrist, and connect the alligator clip to the metal frame of the open computer case,
2) You simply touch the metal frame of the open computer case to relieve your body of static.
(Again, computer unplugged from power.
Also, IF you get up, and walk away, when you return TOUCH the metal frame again)
This is Dell Support, and the Service Manual for the Dell Dimension 8200 desktop computer, http://support.dell.com/support/edocs/systems/dim8200/
To open the computer case:
Click on Opening the Computer Cover
The computer cover opens like a clamshell.
Observe where the cables (Wires) go to the motherboard, from the Front of the computer case.
You may wish to make a drawing, and notes.
Dell DOES NOT state where the wires go.
If any wires are accidentally bumped loose, you will have a schematic made by you, of where they go.
As you proceed, state any problems you are having, and let's see if I can rectify them with information. Simply state in a Comment.