It's changing because when you ask Windows to open a .mak file and it presents you with a list of programs to open the file with, when you select Notepad, you have the "use this program to open this file every time" box checked which permanently asks Windows to associate .mak files with Notepad. If you opened Notepad first, and clicked File->Open and selected the .mak file, it would not associate to notepad files. You can also add Notepad.exe to your "Send To" menu, and then right click on the file and send it to Notepad, which would also avoid associating the file.