I've just tried to re-animate an old P3-933 MHz which was known to be running just before it arrived at 'Das Werkstatt'. I've plugged the most vital components in: CPU, RAM, Graphic card, Keyboard - and attached a monitor.
Well, I've flipped the switch and - nothing happened.
I've tried a different graphic card, different RAM, different power supply. no difference. still wouldn't even beep.
I've somehow given up already, but I just let it rest and went to see a movie. Afterwards I tried again, BUT out of some weird desperation, I unplugged the keyboard and whoop: it gave me a nice pic on the screen, telling me that the keyboard's missing (what a surprise...)
I suspect that it was the polyfuse on the board (GA6VX7-4X) which needed some time to reset its state. As a matter of couriosity, I've re-attached the keyboard again and it didn't do anything again - as expected.
Now I'll have to wait for the polyfuse to reset again (I hope it will)...
What are polyfuses?
http://www.wickmannusa.com/html/pptcsm.html