In a simple circuit you can have a variable resistor which enables the person to control how much power is going through the circuit. I want to be able to replace this variable resistor with a computer (possibly using parallel or serial ports). I just didn't know whether it would be possible.
I'm pretty sure that can't happen. You'd have to be using a circuit that can utilize computer signals. A computer-controlled resistor would work, but I doubt you can control the resistance of computer signals from a port by itself.