I think what is a power supply problem... or a cable making false contact.
Open your case an chek and adjust all the power conctors.
If no work, disconnect all, exept the mainboard connector, and unplug
all of your expansion cards, like sound card, modem, etc.
If no work, disconnect the IDE cable from the mainboard a try again.
If he mainboard starts up, but the vga card not displays nothing, unplug
this, an plug again. Be sure of no are a false contact between cards and
pci or agp slot.
If no work, try to unplug and plug again the cards of RAM.
Be carefully with static!!!!!
At last, try to reset the BIOS chip with the jumper or extracting the battery.

If you cannot start up your system, please emailme or add to my msn.
I can help you.

Good luck!!!

may the force be with you!!!