Building a computer, but it won't boot

I'm building a computer, it's all built, but the motherboard won't POST. the system status light on the motherboard is on. Here's the weird thing, when I hit the power button, all of the fans move for a split second (like a twitch), and then go back to still. All the power plugs are plugged in, and I already tried resetting the CMOS. My next step is just to take it all apart and rebuild it from scratch, testing it as I go.