There was no need to post a new question, an admendment to your original question (by posting an answer yourself) would have done..
But now we are here :)
the first thing that a lot of people do is they run the PC output level far too low so the volume on the external amp has to be turned up to compensate which introduces a lot of noise, turn the amp right down to start, open the volume control on the PC and turn everything up full and then slowly turn the amps volume back up and see if that improves things.
If not another thing worth trying is, again, open the volume control on the PC and put a tick in the mute tick box of everything that is not being used (if your playing MP3's then all but wave) because it may be one of the other sound channels that is picking up the interference.