My Reply #16 above should clear that up, see extract of that post.
http://forum.avast.com/index.php?topic=145084.msg1053949#msg1053949

You should only use [b]notepad[/b] to edit this and [b]you should run notepad.exe as administrator[/b] in win7 or higher.

You don’t say what the error is, nor do you say what your Operating system is ?
Both of these can help us resolve your edit problem and you should only be using notepad.exe for the edit.