How is your pc mounted?
If it is in a metal box. just diode up the pos and neg, with apprpriate capacitors as DC or AC blocks to stop interferance.
if it is not in a metal box, get some tin foil, ine the box it is in ground it and apply the filters as i before, there is always a chance of pulses from start ups travelling through systems and fuses, diods and capacitors would be my suggestion.