My father's given me a desktop computer to reformat. Every time I press the power button, the computer flips off after a split second. In order to turn it back on, I need to flip the main power switch in the back and wait for the light on the motherboard to go off. I then switch the main button in the back and then the front power button on, but the same thing happens. On a few rare occasions (maybe two or three times) the computer actually stays on. I don't believe that this issue is coming from the motherboard because as long as the computer's plugged in, there's a green light that stays on. My father says that he never heard of this computer doing this before (but he has a large office where several employees get their own computers to work on), so this very well may have been happening for a long time. Can anyone offer any help? It would be greatly appreciated. Thanks.
By the way, I know absolutely NOTHING about hardware, so let's try and keep things simple. 
