Compaq 247320-003 - Deskpro 4000 - 5166 Model 2500/CDS Reference Manual page 86

To install optional equipment, you must remove the computer
cover to gain access to the drive bays or expansion slots you
want to use.
To remove the computer cover, complete the following steps:
If you have not already done so, you will need to remove
the shipping thumbscrew/plastic pin from the center of
the rear panel before removing the cover.
1. If you have locked the Smart Cover Lock, use Computer
Setup to unlock the lock.
For more information about Computer Setup, refer to
Chapter 8, "Using Compaq Utilities."
2. Turn off the computer and any external devices.
3. Disconnect the power cord from the power outlet.
4. Slide the cover latches on the sides of the desktop computer
toward the front of the computer.