Some parts are not designed to be removed from the case.

The motherboard has quite a few pieces of hardware.

Mini Tower Computer - Documentation

http://support.dell.com/support/edocs/systems/opgx620/en/ug/.../tindex.htm

Replacing the System Board

Dell™ OptiPlex™ GX620
User's Guide

Removing the System Board: Mini Tower, Desktop, and Small Form Factor Computers

1. Shut down the computer through the Start menu.
   
   
2. Ensure that your computer and attached devices are turned off. If your computer and attached devices did not automatically turn off when you shut down your computer, turn them off now.
   
   

NOTICE: To disconnect a network cable, first unplug the cable from your computer and then unplug it from the network wall jack.

1. Disconnect any telephone or telecommunication lines from the computer.
   
   
2. Disconnect your computer and all attached devices from their electrical outlets, and then press the power button to ground the system board.
   
   

CAUTION: To guard against the electrical shock, always unplug your computer from the electrical outlet before removing the cover.

1. Remove the computer cover:
   
   
   • Remove the mini tower computer cover. 

     <ADMIN NOTE: Broken link has been removed from this post by Dell>

     
     
     
   • Remove the desktop computer cover.
     
     
   • Remove the small form factor computer cover<ADMIN NOTE: Broken link has been removed from this post by Dell>.
     
     

NOTICE: Before touching anything inside your computer, ground yourself by touching an unpainted metal surface, such as the metal at the back of the computer. While you work, periodically touch an unpainted metal surface to dissipate any static electricity that could harm internal components.

1. Remove any components that restrict access to the system board (CD/DVD drive(s), floppy drive, hard drive, I/O panel (except for small form factor computer).
   
   
2. Remove the processor and heat sink assembly:
   
   
   • Remove the mini tower processor and heat sink assembly.
     
     
   • Remove the desktop processor and heat sink assembly.
     
     
   • Remove the small form factor processor and heat sink assembly.
     
     
3. Disconnect all cables from the system board.
   
   
4. Remove the screws from the system board. See the illustrations below for screw locations:
   
   
   • See "Mini Tower System Board Screws" for an illustration of the screw locations.
     
     
   • See "Desktop System Board Screws" for an illustration of the screw locations.
     
     
   • See "Small Form Factor System Board Screws" for an illustration of the screw locations.
     
     
5. Slide the system board assembly toward the front of the computer, and then lift the board up and away.
   
   

Mini Tower System Board Screws

1	mini tower system board
2	screws (10)

Desktop System Board Screws

1	desktop system board
2	screws (10)

Small Form Factor System Board Screws

1	small form factor system board
2	screws (8)

Place the system board assembly that you just removed next to the replacement system board to ensure it is identical.

Replacing the System Board: Mini Tower, Desktop, and Small Form Factor Computers

1. Gently align the board into the chassis and slide it toward the back of the computer.
   
   
2. Replace the screws on the system board.
   
   
3. Replace any components and cables that you removed from the system board.
   
   
4. Reconnect all cables to their connectors at the back of the computer.
   
   
5. Replace the computer cover.

   <ADMIN NOTE: Broken link has been removed from this post by Dell>

   
   
   

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