My problem occurred a few days after installing my new MSI HD 6870 Hawk and Corsair CX600 600 Watts power supply. I had to move my table to another place, so the first thing I did was to turn off the switch behind the power supply. Then I unplug everything including peripherals. When I plug in just the power cable to a power outlet directly and then turn on the switch behind the Power Supply, the computer wouldn't turn! The lights and fans wouldn't turn on. Exactly nothing happened when I press the ON button. After an hour, the computer suddenly turned on by itself and everything worked fine. There were no errors in Windows and my games played fine. This happens to me every time I replug the power cable or turn off and on the switch behind the PSU. My MSI HD 6870 are 38C idle and maximum of 53C while loaded. I've never touch any configurations on my GPU. Thank you.
Next time after you switch off the PSU at the rocker-switch or unplug it from the power point, press and hold the computer's start button for 20 seconds to purge the PSU capacitor. Then release it.
Now when you restore power to the PSU again and you press the computer's start button, it should start up straight away.
There is no need to switch off or unplug the PSU everytime after you've finished using your PC, no need at all. just shut down your PC and leave the PSU plugged in and switched on, it's quite safe like that.
You only need to unplug the PSU or switch it off at the rocker switch when you are going to to do some maintenance work or upgrade work inside the case, or you are moving it to another location.