You can use VC++, but I use C#. You need to look at the
System.IO.Ports namespace on MSDN. You are going to want to use the SerialPort class. A couple of examples (in C#) can be found
here.
As for the UI, the windows System.Windows.Forms library is very easy to work with and has lots of controls. You can use the Trackbar control (which is the slider) to get input values and then send them to the RC with the serial port.