Power unplugged from computer, and all cables;
1) Look at the back of the computer. ONE of the Side Panels remove.
There will be screws vertically along the edge of the Side Panel, that removes. Either Philips head screws you remove, or Thumbscrews that you just loosen.
Once screws are removed, or Thumbscrews loosened all the way, look for a bulged formation in the back of the Side Panel.
This is a fingerpull.
Pull back on the Side Panel. It may only pull back so far, then stop.
This is because the Side Panel has hooked tabs that protrude from it, and go into slots in the metal frame of the computer case.
The Side Panel is then slid forward a tiny bit, and removed straight out from the computer case. You have to get the hooked end of the tabs, to clear the slots in the metal frame.
Now FOLLOW Anti-Static Precautions!
Your body carries Static electricity. Static WILL fry out (Short Circuit), the delicate hardware components inside a computer.
Relieve your body of Static BEFORE reaching inside your computer, and/or before taking any new parts out of their anti-static bags, or cartons.
Computer On a table, computer unplugged from power, computer case open;
TOUCH an unpainted surface, of the metal frame of the open computer case.
This action will relieve your body of Static.
IF you leave your computer in the middle of working on it, be SURE to Touch the metal frame again upon your return.
I then lay the computer on a static free towel, on the table; and computer case opening -> UP.
Makes it SO much easier to work on.
2) Next remove all power cables going to the motherboard, and peripherals. (Harddrive, Optical drive/s, Card Reader, Floppy Drive, etc)
Flop, and/or tie the power cables out of the way, to the outside of the computer case.
If the Power Supply power cables are still in the way, remove the Power Supply.
Look around the outside edges of the motherboard, for motherboard mounting screws; and to the inside.
When removing the screws look to see if fiber washers are used.
Save these washers. The new motherboard may need them also.
(Rule of thumb is;
If the motherboard mounting holes have NO metal rings, fiber washers are used.
Metal rings? No fiber washers )
The motherboard is tilted up to remove. Tilted up on the OPPOSITE side of the I/O area side.
This is the area on the back of the computer, that the mouse, keyboard, monitor, audio cables, etc. is.
Components plugged in here plug directly TO the motherboard.
The metal shield that surrounds the I/O area, is the I/O Shield.
If installing a new motherboard, and it is not an original OEM motherboard replacement, use the I/O Shield that comes with the new motherboard.
That is; IF it comes with one; and IF it fits the rectangular hole in the metal framework; of the computer case.
Dell Support shows no documentation; and reviews are sparse.
WAIT, hold the phone!http://www.ebay.com/itm/Broken-Dell-Dimension-L600R-MCM-Computer-Desktop-Parts-/140517075990
OH, I see now said the blind man!
The lever on the Top/Back is depressed, or pulled up on, or pulled back on. This releases the half side of the computer case; that removes.
The front of the computer is the Front Panel.
The area of contact pins on the motherboard, that the main wires from the Front panel go to, is the Front Panel header.
Power On switch, Power On LED light, HarDDrive activity LED light, and Restart switch; IF used.
BEFORE you do ANYTHING, make notes, and a drawing as to what pins on the motherboard; that those wires go to.
I DO NOT have a pinout chart available for the Front Panel header.
Power cables from Power Supply I can guide you on.
For additional questions, (And I expect many), please post in a Comment.
Happy New Year to you, and yours!