A circuit adopted in a computer includes a power supply (20) with an output (PS_+5VSB) for providing a standby voltage, a motherboard (10) with a pin (MB_+5VSB) for receiving the standby voltage, a switch circuit (30) connected between the output of the power supply and the pin of the motherboard. The switch circuit has a control terminal fed with a S5# signal which is at high level to turn on the switch circuit, and at low level to turn off the switch circuit when the computer is in a S5 power state to cut off the standby voltage supply from the power supply to the motherboard for reducing power consumption of the motherboard; the control terminal of the switch circuit further being supplied with an inverted PWRBTN# signal for turning on the switch circuit when the PWRBTN# signal is at low level for switching on or off the computer.