Notepad, by default, save files with the .txt extension. If you open the file and use "Save As", then select "All Files" as file type, it will preserve your .par extension, otherwise you'll get mymodelname.par.txt as the actual file name. If you're only concerned about the .par file being "Associated" with the NotePad application, there's nothing to worry about, just change the association within Windows Explorer to your preferred .par editing application.
Here's a Windows explanation of how to change file associations with WinXP.
http://support.microsoft.com/default.as ... duct=winxp
-- ggunners