Brand spanking new computer and no power

I have just put together a new computer with a
iwell moptherboard, AMD processor, new video card, hard drive from old computer, modem from old computer, nic card for cable modem from old
computer, new RAM, new case and power supply.

I plug it in and the fan on the AMD processor and the power supply run for about 2 seconds
and then nothing. I have tried pulling each card seperately and then turning it on and still nothing. I finally reseated the RAM and
it started up, but it was not hooked to monitor or anything else. So when I sat it back down on the floor I decided to try the
power one more time, and NO POWER!!

What gives, any ideas?

Mark