Had a similar problem not too long ago - fans on PSU came up to cruise speed - disk activity light blinked a few times ... But no video or keyboard!
Turned out that the small fan mouted over the video chip and the Northbridge chipset on the mobo had quit - followed shortly after by the chips it was supposed to be cooling ... Had to scrap entire system as replacement mobo no longer available for that particular Athlon chip ...
Check whether you have such a fan (mounted more or less in middle of mobo) - and then check whether it is actually spinning.