Sorry, I thought you were running your PC to an amplifier, not to a head unit. That head unit doesn't draw much, and they tend to be much better grounded and shielded against RFI. Your computer, on the other hand, is not and can pick up all kinds of nasty noise. Between that and the additional current, that's why I recommend using a dedicated 12V line to power your PC.
If you have any kind of amplifier installed in your car, you should run a new line for it.