The best way I think would be if the application you want to get data from is COM-based. Then by using CoCreateInstance from the application that needs the data, COM would connect to the other application if it is already running, or start it up if it was not. Then the interface you get back from the function call would allow the two applications to communicate with each other.
Another way you could do this is by having the application you want to get data from create a named pipe or event. Then the other application can attempt to open the pipe or event. If it opens, the other application is running, and if you use a pipe you could even use it to pass data between the two applications. If the pipe/event doesn't open, you know the other application is not running, and you can then start it from there. This is because named pipes/events are automatically deleted once there are no more applications running that have handles open to them.
Another thing to try, if the applications create top level windows, is to have the application needing data enumerate all the top level windows, using the EnumWindows function. If you find a window with a title or class information that matches the other application, you know that application is already running, and could even use its window handle to post messages to it.
