interference / noise
Along the same tracks as Lzealot, try plugging your computer into an outlet on a different circuit in the house. Some major appliances, especially older ones, may "transmit" some type of stray electrical charge back through a circuit.
Refrigerators, microwaves, drills, A/C units, washing machines, dryers, dishwashers, heaters, etc., may all cause that type of interference. UPS's and surge suppressors may also cause electrical interference if something is wrong with them. So if you plug in via one of those, try using a different one or try plugging directly into the socket on the wall with the suppressor unplugged. Fluorescent, and other types of other-than-incandescent lights may transmit a type of interference without being on the same circuit. Think about fish tanks, overhead lights, and some types of outdoor lights that may use a circuit near the same one that you use for the computer.
If you still have the interference on different circuits, then take it to work and try it there, or visit a friend that lives in a different neighborhood to see if that same noise is present there.
You basically want to rule out anything external before you cave in and buy another power supply.