PC Hardware

Hello,My desktop (hp,e-vectra) can`t power up.i can see a green light and then nothing.
checked IDE cable looks good,power cable works fine too.on pressing power button,Process Runs for a Second and Hard drive Spins as well, but suddenly it shuts off and I have to unPlug and Plug it back in to test it again.No beeping or anything on monitor.What Could be the Problem, which Compenent is bad if anyone can help – with thanks !