USB Power -

Does the USB (1 or 2.0) spec say anything about being able to control
the voltage on the V-out wire in the USB cable? Eg: Can I set the
voltage to any value between  0 and 5V?

Can I do this through a software API (for Windows)?

Is it possible to control load current this way (instead of voltage)?

you use parallel ports or serial ports.
parallel port is better than serial port.
if you want to use USB ports definitly, you can use USB to parallel or
serial converter.