I agree something is amiss but here are the most common reasons. Look at the pictures in the first supplied link and instpect your machine.
2. A machine with an aging power supply could do this as well. Many can't accept that 350 Watts is not enough or the unit decreases in capacity with time. I've written at length about this and will not do so as much. Your choice to look into this.
My desktop which is just over 2 years old takes about 4 times or more to get started. I have to keep pressing the reset button. I have reformated the drive in case it was a trogen or virus. I then have reinstalled windows xp and also tried putting a ghost image back on but it made no differance. each time the error message can change but after 4 times or so it does start. the stupid thing is that once you are in and then get and press restart it will start no problems no matter how many times you do it. but leave the computer off for 30 minutes or so and then try and turn it back on you need to press the reset button again 4 or 5 times. does anyone have any idea what it may be. i think it may be my hard drive on the way out. it is a 80GB SAMSUNG. ????