I'm going to go ahead and assume that by "assign it", you meant "open the file in Notepad" and not "edit the registry to associate txt files with Notepad". Even if that assumption is correct, no. Just no. Don't override people's preferences for text editors. If the guy wants to view txt files in Notepad++ instead of Notepad (and who can blame him), then let him view his txt files in Notepad++.
in reply to Re: Open txt file using default program on Windows
in thread Open txt file using default program on Windows
I use system ("\"$filepath/log.txt\"") to open log files in case of errors. It seems to work OK.