Check your Motherboard for any capacitors that are bulged, burst or leaking liquids. I had the same thing happen to me, and it was just one tiny capacitor called it quits.
The only solution then is a new motherboard.
By the sounds of your machine it is a few years old, and from reading an article, which I can't find at the moment, there was a problem with Motherboard capacitors between 2000 and 2004, where capacitors were made cheap without the correct recipe for the goo inside of them, and after a number of start ups and shut downs they would burst or bulge, effectively killing the computer...