Radio interference... just Google search for info, you might be able to pinpoint how to fix it.
I used to have this problem on my budget speakers but usually higher-end equipment is shielded from it. I could actually hear telephone conversations as well.
If they are plugged in to the wall, it can even happen when your speakers are not plugged into your PC.
(As a funny side-note: the image on my PC's monitor used to vibrate randomly, like the monitor was failing. Then it would suddenly go back to normal. It took me a few weeks to realize always this always coincided with my roommate using the microwave in the kitchen, directly across the wall from my computer. I just moved the monitor to another side of the room and the interference stopped.)