Powers on and turns off 10 - 20 seconds ?????

Question

i have an e-machines 610 and my power supply is a bestec atx-250-12e...the problem i am having is that when i power the pc on the green power light comes on and flashes, then after about 10-20 secs it turns its self off ??? during the time the pc has power i cant even open the dvd drives basically no power is running to the drives..ive even tried just putting power to the drives and still no success in opening the drives...

do i need a new power supply or do i need a new motherboard ???

thnxs for any help ****