PC power starts then stops immediately

I have a Asus M2N-MX motherboard with AMD Athlon 64×2 4800 with 2 gig of ram. When I power up the pc tries to start but the power supply shuts down. I have stripped all power off and the only thing running is the mobo but the power supply still shuts down. It is only a few months old any suggestions???

Thanks in anticipation