If you mean computer fan, best solution is to have it in another room and stream wirelessly (eg Squeezebox or apple airport express)... Although if the noise gets significantly quieter when you put your hand on the case, a few squares of high mass acoustic matting (like used in car soundproofing) works wonders. If you mean any of your actual hifi/av boxes, a rack with doors is probably the best bet, but have to be careful about overheating.
And if you mean the excessive noise a Virgin V+HD box makes even on standby, I'd love to know too!!! ;-)