Have you tried plugging in your computer into another outlet where there is no electrical interference?
Here is a web site that might help you better understand the conditions or problems you might have.
http://www.avdeals.com/surgesupp/monstersurgesupp.htm
I think you can find something similar to the items mentioned at the website elsewhere, somewhat cheaper.
The computer is picking up noise from where your power supply is plugged in. Something else, electrical, could be interfering with your audio signal.