Ragarding the PC and burned traces, sound like a fuse would be a great place to start. Then i'd move to a small resistor to measure the ground current. Then i'd investigate why i had ground current of enough magnitude to flash the trace.

I have done quite few Parallel port interfaces most "stole power" so i did not have to worry about the mutual ground between a PC and a connected device in those cases. However, i have connected independantly powered devices to the Parallel port (as people do everyday) and never had a problem. Check the ground differential and ask yourself why.

regards,
p