This looks like the avast5.ini file is only being opened with read permissions (a self-defence module protection), something which happens when you use another text editor isn't meant to happen with notepad.exe.
It certainly doesn't happen with me editing avast5.ini using notepad.exe and XP Pro SP3, so I don't know if MoinMax is using Vista like you or win7 and if UAC might be getting in on the act. If it does and is running notepad as a limited user, I don't know if this might be an issue.