Radio interference

hi all, Iv'e just bought a 2.1 speaker set for my pc, i was previously using a small hifi as my audio equiptment but found i got alot of radio interference so i purchased some new equiptment, after id set it up i noticed i was still geting quite abit of radio interference, there is no ground connection on the main unit so i cannot earth it. i tried earthing my last set up and it didnt really make a difference, i think the interference is being picked up by the tower because it stops when i remove the sound card.does anyone have any ideas how i can stop this happening.thanks