Anybody have an idea why, the system power supply works but pc won't respond when powered on? The system fan won't even start. Thanks for your solutions in advance.

If it's an AMD system, I've had experiences in this.. If the PSU is good (You've tested it 100%), then try the motherboard. Been inside the PC lately? You probably put a teeny tiny charge on a transistor or CPU, causing a .5 second reaction to the motherboard. IE, static stopped you. IT's tough.. But that's usually the answer. Try the CPU in a different mobo, try the mobo with a different CPU (Test CPU first, don't put a good CPU into a bad mobo = another KIA CPU if you do). I hope I helped.. If that's not the case, you may have overclocked it a teeny bit too high. Clear your CMOS and restore all of your settings, that should help you. Good luck, hon. Feel free to mail me if you think that's not it, I'm sure we could get it figured out..