LabVIEW
Control a serial port pin for a variable DC output.
I am also of the opinion of just getting a simple USB DAQ with analog outputs. Looking at NI's Entry-Level DAQs, the USB-6001 will probably do the job. Of course, do go through the specifications to make sure. But at $224, it would sure beat the cost of circuit design (this alone would probably be more than the cost of the DAQ), buying Printed Wiring Boards (PDWs), populating the PDW, and verifying the circuit.
