You can do it in a number of ways, but essentially your PC is going to open a serial connection to the Arduino and send data to it which will be interpreted into state changes of the digital ports of the Arduino. The content of the messages is up to you, as long as both ends know what it means.
Step 1 would be to learn how to drive the motors on the Arduino by writing a program that drives the correct ports.
Step 2 is to modify that program so that when you type 1180 in to the serial monitor the Arduino turns motor 1 180 degrees, etc.
Step 3 Start Visual Studio, press F1 and search MSDN for a Serial console program that you can 'borrow'.
Step 4 Check your new serial program can replace the serial monitor and you can still turn motor 1 to 180 degrees.
Step 5 Put a flowery GUI over you serial console and polish your turd. :)
It doesn't matter what the GUI does, its still just a serial client talking to the Arduino. You could offer Bluetooth or Wi-Fi connections, but its essentially the same beast.