I was starting up a pc running xp. during the booting process, everything went dead and would not start back. I concluded it was the power supply. I happened to have another 300w supply on hand. I hooked it up and started it. The computer began to boot up and in the middle of doing so it died again. Now it wont start with either supply. What can cause this? The fuses on the supply is in tact. It shuts down "completely" , just before windows appears.