Baffling Black Screen During Boot

Not a hardware guru and seem to have hit the wall on figuring this one out. Need a fresh viewpoint.

Dimension 8300 (old but doing all I need -- have a much newer machine for higher end apps.) Fully up-to-date XP.
No recent changes other than XP updates.

Away from it when something happened. Screen was black but the machine was running.  Hard reboot got to Starting Windows then went black. Subsequent reboots sometimes did not get past POST but then made it as far as the Windows password request.  Each time ended in a black screen. (Was trying to see if a Windows Update occurred that needed several reboots to complete.)  Eventually got stuck steadily before Windows even started. Boot message would come on and the screen would black too fast to actually read anything.

Disconnected all peripherals including hard drive and opticals, except mouse, keyboard and GPU. Swapped mice, keyboard and GPUs in turn without a change.

Fearing a dying motherboard I examined it closely and found a row of leaking capacitors.  I purchased and installed an exact duplicate of the motherboard, but the problem persists.

The power light stays green.  The status lights, A-D, show Green Green Green Amber. I hear two quick beeps after the initial boot message. I can hit F2 to start the System Setup, but that screen shows for only a second before it goes black.

The screens are:

Dell splash screen, then the two beeps then

    Phoenix ROM BIOS PLUS Version 1.10 A07
    Copyright 1985-1988 Phoenix Technologies Ltd.
    Copyright 1998-2001 Dell Computer Corporation
    All Rights Reserved

    Dell System Dimension 8300 Series
    BIOS Version A07
    www.dell.com

    Diskette drive 0 seek failure
    Keyboard failure

    No boot device available -
      strike F1 to retry boot  F2 for setup utility

There is no diskette drive...never was.

The keyboard works to hit F2...though it is a USB keyboard and the system originally had a PS/2 keyboard.

The monitor power light stays blue indicating there is a signal (it goes amber if the signal is lost.)

I suspect the BIOS is confused and could be resolved in System Setup if I could keep it on screen.

Thoughts on how to get there?

thanks!