PSU actuall shot sparks out back past fan. I replaced it with a new one. With the PSU in the OFF position, the Power light pulses. Turn the PC on and everything works perfectly - boots with no errors. Does the ON/OFF button on the front of the case actually switch the PSU or what? While the Power light is pulsing, it is difficult to get the PC to recognize the ON/Off button, but it eventually does work by holding it in a few seconds and then releasing. XM

Your mobo was probably damaged durring the "explosion." Your switch should be a problem unless you somwhow hit it hard or something. The switch tells the mobo to connect the green wire(forget what voltage) to the ground wire. Thats what starts the PSU. Sapphire PC-A9RD480Adv Sapphire Radeon X850CF AMD Athlon 3700+ 2.63GHz 1024MB512x2DDR400 RAM 350W Acetech PSU 16A;12v 300w SPI PSU 18A;12v

To add a little to what Ian209 said. The power supply always supplies 'some' power to the motherboard (as long as it's plugged into the wall). The power switch tells the motherboard to let the power supply power up all the way. That's as long as the motherboard senses that all the voltages are within specifications. Do yourself a favor BACKUP!!!

Seems like your mobo was damaged when that happened, because your power light shouldn't pulse like that if your pc is in the off position. Only pulsing that is normal for a powerlight is when it's in standby mode. Some motherboards do this that use a 2 in 1 function on it power/standby light so that it can tell the pc user that their system is in standby mode. Before that most pc's had a seperate light for stanby mode, but most new pc's share the same light as the power button now. If you can swap out psu's for another and see if it still does it then that would be the next bet. if it doesn't then there was something wrong with the psu if it still does then the old psu blew something in your motherboard. AMD Athlon 64 X2 4400+ OC 2.7ghz 2GB Dual Channel DDR 3200 Nvidia 7800GT SATA II 2x 200gig 7200rpm 16mb cache RAID-0 Gigabyte Nforce 4 SLI

Sprry I meant to say some power lights have a 2 and 1 function to it on the case. AMD Athlon 64 X2 4400+ OC 2.7ghz 2GB Dual Channel DDR 3200 Nvidia 7800GT SATA II 2x 200gig 7200rpm 16mb cache RAID-0 Gigabyte Nforce 4 SLI

Thanks guys. I put in another PSU just to be sure and also checked the ON/OFF switch to the motherboard. Regardless of the PSU, it still pulses and it is not in STBY. I guess it's not a minor problem after all! The system is too old to replace the motherboard(2001) unless I can run across someone with an old Shuttle AK31A. Thanks again.