How do I know if the power supply is bad or the motherboard is bad? Or maybe some other component is bad?

Ok, I have a Dell Dimension 2400. When plugged in the green standby light on the motherboard comes on. When you press the power on though, nothing happens. I've tried other ATX PSU (known to work), but still get nothing. I've tried byping the power on, but jumping the connections...still nothing. My latest attempt was to put a jumper wire in the back of the 20 pin connection between the green wire and the ground....the results were: PSU fan came on, and the CPU cooling fan came on and one of the LED's of the front panel came on (identified as the HDD activity LED). However, at this point the HDD was not even in the case (I was retrieving pictures, since I could not get the computer to even boot). Since the fans came on I put the HDD back in, but though I got the same results, the HDD didn't seem to spin or show any other signs of working (I was able to hook it to another comp. to retrieve pictures, so I know it is functional). Just wondering if there is another way to test? Should the jumper to the 20 pin connection rule out a bad power switch? Any suggestions? Thanks.