Sounds like a simple application. Just get a 5v relay like this maybe: http://www.allelectronics.com/make-a-store/item/RLY-499/5-VDC-SPDT-6A-RELAY/1.html
I would wire the relay coil via a spare USB cable, just hack of the end and wire the USB +5vdc and ground wires to the relay coil and plug into your PC. You might want to wire a diode across the coil terminals, anode to the ground wire, cathode to the +5vdc wire, to clamp transients. The six amp contact ratings for that relay should be able to handle just about anything you might have in mind.
Or you could wire the relay coil to a spare internal disk drive power connector's +5vdc and ground pins.