If I were you, I would disregard the above post...
I agree with mektek that this sounds like an interference issue. Are you near any sort of heater, water heater, broiler, radiator, or anything like that? Have you tried moving your computer to a completely different part of the room with a different outlet?
You might also want to try physically moving the speakers away from your computer desk area. Although a bit of a longshot, it's possible that they're not properly shielded. Are these speakers designed specifically for a computer?