When you open this file up, does a small window that says "Open with" with a list of programs pop up? If it does, scroll down in this window untill you see notepad, and then click on it. If you want notepad to always open this extension, check the little box that is below the scroll list if it isn't already. If this little window doesn't pop up when you open this file, I don't know what else to say. I hope my advice helps.