If you were using the web based client then Avast! won't have even noticed you were sending an email. Besides that, the outgoing mail history is the sent folder. If you've deleted it from there then you've effectively deleted the history.
If you were using a mail client on your computer, then you may be able to check the web client to see if the email is still on the server.
You could try contacting the Windows Live support team, but due to privacy considerations they may not be able to retrieve (or even see) any details about what you send or who it was sent to.