OK, it means that you probably don't have notepad.exe - somebody/something has deleted it.
That would explain the behavior of avast! Virus Cleaner - when started, it checks the associations for basic file types, including txtfile. If they are found to point to a non-existing files, they are fixed - i.e. set to default value. Unfortunatelly, the default value for txt file is link to notepad.exe, so it actually isn't changed.
So, you can safely ignore the Virus Cleaner warning. I'll try to fix this confusing behavior in the future versions. (However, it's not very usual not to have notepad.exe... it means you cannot open .TXT files from Explorer now.)