providing input to another process

Create a text file like this:

input_formatoutput_formatfilename.extension

Now execute the command:

converter.exe < text.file

which will use that file for standard input.

Wow, quite a bit easier than I had anticipated. Thanks for the help!

In Windows, you have one of two choices, to use anonymous pipes or a named pipes. Anonymous pipes only allow one-way communication but are extremely easy to use. Named pipe allow for full-duplex communication but are (I believe) quite difficult and burdensome to use for simple two-way interprocess communication. My suggestion is to use two anonymous pipes, one for each communication direction. MSDN's section on anonymous pipes should get you started on creating and using them. There's an especially useful example linked at the bottom of the page called Creating a Child Process with Redirected Input and Output which is exactly what you want.

This is the general solution when you want to create a GUI frontend for a console application. You could also create a file and just redirect input as stylin suggested, however that's more for console applications that just do quick data processing and don't really drive the GUI. It looks like that's what you wanted though, so good luck to you.