Hm, thats not so much car audio as it is a power issue.
It could just be a weird loop when your HU is trying to draw a signal from your PC when it is off..it just gets a buzz.
Only way I can see that being fixed for cheap is to go to radio shack and buy a transistor. Then splice into the wire going into your auxiliary input and place the transistor there.
A transistor has 3 connectors. Positive, Negative, and a switch. You would wire the switch up to your PC's Power supply.
I made a diagram. You would probably need a resistor to keep from blowing the transistor.
A transistor is really tiny. I just made it large in the image for viewing purposes.
It just acts as a switch. It will not send any signal through the wire (as if nothing was connected at all), until the PC is turned on.
Now, I have no idea if this would actually fix your problem, or if there is a better way. But is is an idea on a possibility.