If the computer starts to load, meaning, showing signs of life then stops, it might be some wrong setting in the BIOS. Please set the BIOS to default values or, even better, reset the BIOS. If you need help with that let us know.
Could you tell us what happens exactly? What happens and at what stage of the loading process the computer turns off.
If there is no response at all when you press the power button:
1: It could be a faulty power switch or loose connections of the power switch to the motherboard. If you are capable of doing this: open the box and make sure the connectors are well secured to place. Even better, remove the power switch connector and try to short the two pins on the motherboard with a thin screwdriver. If you manage to start the computer each time then your power button is faulry.
2: The psu is triggered by the motherboard when the power button is pressed. Please do the following: disconnect the large connector that connects the psu to the motherboard. Reinstall firmly. Repeat this 2-3 times to make sure that if there is any oxidation or dirt it will be rubbed off. Do not use exessive force. At the same time make sure that the 4 pin square connector that supply the motherboard with 12 volts is well secured.
3: Make sure the reset button is not partly pressed in.
4: Like others before me said, it could be a psu which is in a marginal state.
Edited by The Skeptic, 13 December 2007 - 10:32 AM.