Have had the same problem twice. First with a Asus P5E SLI based system after through checks of Motherboard PSU ram Graphics and Monitor found out that the culprit was a screw which when tightened to the cabinet caused sone shortcircuiting. The system booted fine outside the case!
The second PC was a Intel Original 925XBC engineering sample I had procured since 2004 2005 in that case the scrutiny revealed that the board had died. Replaced it with a ASRock G41 based motherboard for 2700 in 2010 Previous processor was a LGA775 P4 3Ghz lated changed that to C2D 2.9 Ghz!
The second PC was a Intel Original 925XBC engineering sample I had procured since 2004 2005 in that case the scrutiny revealed that the board had died. Replaced it with a ASRock G41 based motherboard for 2700 in 2010 Previous processor was a LGA775 P4 3Ghz lated changed that to C2D 2.9 Ghz!