I suggest that you don't want to launch Notepad per se, but whatever text editor the user has associated with the .txt extension. Windows, starting at least with Win2k, will open an associated application when the data file is executed. Therefore, the following opens Notepad with filename.txt loaded on most systems:

sub { system 1,"filename.txt" }
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Of course, you'll want to replace "filename.txt" with a variable or function call or something to determine which filename you'll be opening.