I built a computer, and everything seems correct, but it will not boot up. If I press the power button, the system LED lights up for a second, and the fans spin for a second, but then everything powers down. Is there anything likely to cause this?

1. bad PSU
2. jumper not set correctly
3. CPU not seated properly
4. CPU heatsink not affixed correctly
5. [...]

That could be anything in all honesty. For example you could have static discharged into a component when grabbing or going to insert it.. You could have done the same to the whole board zapping it. The processor itself could be bad as well. My suggestion is to take all of the components out of the expansions slots and try reseating the processor and video cards leaving them as the only pieces of hardware in the system. Reset the cmos data and try to boot it. If at that point it still won't boot I don't know what to tell you except invest in a post card for the future and if you don't have one a good grounding strap.

Thanks. The system now boots, and gets to the setup screen. I switched the plug going to the 12v CPU power. Now I am having a similar problem with the plugs. For a second, during the POST tests, the keyboard's indicator lights flash, but after that the keyboard doesn't work, and doesn't appear to have power going to it.