What happens when you push the power button twice in succession? Push it once, and push it a second time quickly after. If it starts, it has something to do with the startup load not being enough to start the PSU.
I have a VIA C7D embedded platform that does this with a normal 12v Pico PSU with a 102W brick.
|Dual Intel Xeon E5-2620-Xigmatek SD1283 DK-II|Asus Z9PA-D8-HR-05 IFX|WD 250gb Velociraptor SATA3|16gb DDR3-1600 ECC RDIMM|WD Blue 640GB SATA2 x2|Logitech K750|Logitech Wireless Mouse|nVidia GTX660ti 2GB|Antec HCG-750|NZXT Source 210 Elite|M-Audio ProFire 2626|Art TubeOpto8 with Smooth Plate Tube Swap|Avid Artist Mix x2|
A 9500GT with 1GB of RAM is the most pointless thing since NASCAR.