David Childers recently collected the following data specific to Dells, probably relevant to Dimension 2400 and later.

At startup, there are four LEDs on the back of the systems. These function very similarly to the old POST boards for diagnostics that were used in the late 1980’s and early 1990’s. They will flash amber and green as the system initializes the subsystems:

Light A – Represents the Video subsystem

Light B – Represents the RAM subsystem

Light C – Represents the Data Bus (CPU to System Board) subsystem

Light D – Represents the Storage subsystem

Once all of the lights are green, fundamental hardware failures can be ruled out.

To reset the BIOS to factory defaults:

1. Remove the power cable from the power supply. Leave the power switch on the power supply ON.
2. Press and hold down the power button on the front of the case for at least 20 seconds.
3. Replace power cord.
4. Start system.

To clear the parameter RAM (CMOS)

1. At startup, press F2 (or whatever) to enter the BIOS setup.
2. Once you’re at the main BIOS screen, press alt-f to clear the CMOS RAM

A most interesting BIOS option

You can disable BIOS control of the PCI bus, and set it to OS control.