[snippage]
I should've checked the CMOS battery first. But when I checked it was
dead so I replaced it with a good one. Now the system no longer responds
to keyboard like it did at first. Beep speaker give single beep, OK.
System shuts down unexpectantly after five seconds. This means CMOS needs
tweaking. Currently running with 500 MB RAM module and Q8400 CPU.

Later.

Do you have the heatsink/fan on the CPU ?

Is the cable from the heatsink/fan plugged into the *CPU* header ?

The BIOS expects to find the pulsations of the RPM signal from
the CPU fan, on the CPU header pins. Failure to provide
pulsations on the third yellow wire, will result in the BIOS
doing a safety shutdown.

No other fan header has such a requirement.

Whether it's a three wire or four wire fan, plug it into
the CPU header. Make sure the fan spins at powerup. If
the header has three pins, and the fan has four wires,
it can still work as long as pin1 goes to pin1. The
user manual will typically label the pins, black is
ground, red might be +12V, yellow could be the RPM
(2ppr) signal. The fan pulses twice per revolution,
due to the way the brushless design, the permanent
magnets and the hall probe for mag field work.

Paul

Yes and yes.

The problem seems to be that the BIOS program does not respond to my PS/2
keyboard. I have BT keyboard and mouse. That seems a goofy idea. I have a
USB mouse but no USB keyboard. I suppose device selection can be done in
BIOS program.

Once the BIOS program decides nothing is happening the PC shuts down.

I sent a message to seller. The PS/2 connections go through the IO shield to
the mainboard. It is independent of any connections to MX300-X gaming case.