can a PC parallel port produce interference?
As soon as I plug it into the PC port with no external power, some LEDs glow dimly yet I never asked for this.


This is usually "phantom power": some of the data lines will be high at startup. This puts 5V through the protection diodes on the pins of the ICs through to VCC, powering the circuit. Since it's been through a diode it'll be less than 5V, resulting in dim LEDs.


  The circuits are very close to the computer.


You could rule out direct EMI with an extension cable. But generally the port will only generate interference while switching.