it could be the switch is in wrong on the psu or it could be on its last legs.
try it with only one stick of ram and one hd,no dvd drive no gcard or usb devices, if it boots you need a new psu, i would guess you will need min 350w but you dont want to try to save any cash with your psu as when the giveup they normaly take somthing with them.
if it boots with no gcard,leave it for a couple of min then, alt+f4 then right cursor once, enter, and that will shut down.
semp 3400+ @2.52ghz, akasa evo33 with silver paste, asrock k8nf4g, 2 x 512mb ddr420(2.5-3-3-7), 7600gt core 625mhz mem 840mhz, X-fi extreme gamer, nec ad-5170a + lg gdr8164b, 2 x samsung spinpoint 250gb, 600w psu, phong 2 case, cpu temp(idle - burn) 26c - 32c(fans full speed)31c - 38c(low fan speed) superpi 1m - 38.344 sec(AF4OB221). cpuz