Power Power everywhere..lol
The first rules to working with power is to rule out anything its connected to. I have had in the past a floppy drive that was stopping my computer from turning on just becuase it kept shorting out my power supply. Remember to turn your computer on, you dont need a Floppy, DVD or CD drive or even your hard drive. Unplugging all these drives will allow you to test the power supply more fast. Just remember you still need power to your mainboard. I have also had people's ON buttons on there pc's stick and it makes it hell to turn on. So test if you like or just go by a new Power supply.