ghj290,
What you are describing is a usual behavior of Outlook and Outlook Express. Every time when you don’t put something in the subject bar you’ll get a warning that you didn’t use this bar. So as far as I know, what you are describing has nothing to do with Avast! 4.
I think that is a kind of protection from MS that you don’t sent messages without an subject line and if you want it on some occasions you can do, but it is not recommended. I personally hate e-mails without a subject line to remember what that e-mail is about.
Jan