Looks like about four subprojects to me.
- PC Code/UI
- USB Communication to/from device
- local brains in the device
- mechano-electronic system to implement your pressure control
I'd suggest starting w/ the 2nd item, as some of the others depend on it. You can build your own custom USB device, but from your problem description, I don't think you want to take that on. I'd recommend something like a http://www.ftdichip.com/Products/Cables/USBTTLSerial.htm, which should have available drivers to make things easy to program.
You then need some sort of embedded device to read the instructions coming from the computer and adjust the mechanical valve. You probably need to measure pressure with this device, and then you need to decide whether to implement the control system on this device or send the pressure back up to the computer to implement the control scheme. You will probably need this device to handle digital I/O, analog input, and probably analog output (maybe in the form of PWM). It's likely you will need to build some sort of driver circuitry to enable this device to drive the valve. Chances are, this device will be some sort of microcontroller, either a bare-metal construction on your part, or something approaching a development board, like the Arduino, Raspberry Pi, or Beaglebone Black. The Arduino is pretty much a microcontroller with a development environment and boot loader with massive user base. The last two are both full embedded computers on a board running linux.
Then, of course, you need the valve.