Background application, data to focused window

Hello, I would consider myself a part-time programmer, who can work around Visual Studio and produce relatively simple Windows & CE Apps. I am working with a DLL and am comfortable using that to read my hardware and return the required data.

However, what i am now looking to do, which I have no experience with is; run an application as a background task that montiors the port for activity and on activity return a specific piece of data and populate the currently focused text box of any other application.

I can handle all the hardware and DLL aspects, i'm specifically looking for advice on how to run as a background app and send data to an in focus text box of another application, if no text entry box is in focus the data may be ignored.

I hope that makes sense, in summary;

1. I have code and hardware that detects activity and returns data, no problem.
2. I would like this to continue running when minimised / out of focus (system try or service could come later)
3. I would like the data (string) to populate the currently focused text entry point, if available.

Assuming point 2 is relatively simple, is there a way to send the data from the application as a keyboard entry? so would go straight into Word, Notepad, TextBoxes, anything where the cursor is focused?

Any help much appreciated.

Cheers,

Matt