I have the feeling that to have a small microcontroller connected to the serial port (simple commands from PC keyboard in Hyperterminal) is just more user-friendly approach .. no software for PC required ..
All what you need is to type, say, 001 for 10mV, or 010 for 100mV and so on ..
When you type a command like 010 in Hyperterminal it sends out these numbers in ASCII, so, in this case the sequence that the micro sees is: 30h 31h and 30h ..
As micro you can use a PIC and to generate output voltage you can choose a DAC, digital potentiometer or just PWM on one of pins ..
Personally, I would use PICAXE-08M with its build-in PWM function, and write code in the PICAXE-Basic, which, most likely, would have something like 10 lines ..
If you wish to have a look at similar device, namely 4-20mA tester based on this PIC have a look at: ..
Regards,
IanP