does the box 'boot up' and run for about 10 - 20 seconds and then just shut down?
if thats the problem it is most likely your heatsink or fan stuff, i got a new AMD box and the heatsink wasnt properly attached, so when i took it back they testeed everything and then realised they had done somthing wrong with the heatsink, but im not quite sure what it was they did to it though