pre bios failure

I have a machine that when powered on the fans spin, a light on the
mobo shines and the keyboard lights pulse. The machine is a Dell
GX270 and has the 4 LED diagnostic indicator lights on the IO
pannel. They are all dark. Previously this machine could only be
encouraged to boot by unplugging the power supply, waiting for the
board to discharge electricity (~45 seconds), then plugging it back in
and pressing the start button. The power supply was checked with
some 20-pin tester - all lines appeared to be providing the proper
power. So, my next tests would be to pull unnecessary boards, reseat
existing memory and necessary boards, possibly pull the board to avoid
any grounding issues ..... but beyond these measures does anyone have
other diagnostic checks? Why's the keyboard flashing? Is
anything indicated by the discharge/boot thing?