The easiest ways have already been explained, saving to a temp file, using the clipboards etc... To get more detailed then that, you have to get into using API to find the proper window handles and set text to that handle. This can be done but doing a quick search Ive seen you have been asking the same question on other message boards for months now and have gotten the same answers including full API examples. Your complaint there was about displaying data to multiple versions of notepad all without saving. Of course without saving there is no difference in the notepad name/title to specify which window handle to use.
Personally I dont understand the need for this in the first place. If the data is already displayed in your custom program why do you even need to display the same data in notepad? Since the answer has already been given to you, I wouldnt waste any more time providing examples without a clear understanding of it all.