How old is it? Newer models let the processor turn off the power supply. Older models the power supply and fan runs even if the board is dead. If it's old and the Power supply fan doesn't run when you turn it on then I'd guess power supply. You said you checked connections. Does that mean you took the case off and checked the power to the drives? If so do you have a long power connection to the mother board and another 4 pin plug to the mother board? Is the little red LED in the power supply or on the Motherboard?