Even after your detailed explanation the nature of txt files is every single byte in it is the text, there is no formatting, no font, no margin encoded in the file.
Therefore what you see in notepad only depends on notepad configuration. Dan guessed this configuration is stored in some registry key. That might be the case. All I can say is, if you go into the format->font option of notepad and choose another font, that will be used for display. And not only for that file. The file isn't altered at all, it's merely a display configuration of notepad and has nothing to do with the file itself or its generation.
So, if you can go back to an older system, just look what is configured for the display font there, and set your notepad the same way.
Maybe you need to choose any monospaced font, like Courier instead of the Win7 default of Lucida Console. I think in XP this was Courier New.
As you talk of margins: In the notepad window you will have no margin, unless your application writes out spaces, and then surely a monospaced font would help, but even in a normal proportional font the same number of spaces at the left side cause the same margin width throughout all lines. Notepad doesn't display any margin area at all, if there is no margin added by empty lines and spaces, the only margin options you have in notepad are about printing with margins, and you also set that up. Those settings are not preserved per file, they apply to any file you open in notepad, also generated by other applications.
Bye, Olaf.