Incoming emails disappearing

Using Avast 4.7 home edition and XP Microsoft Outlook, some incoming emails disappear before i receive them! My Avast tells me that they have come in and who they are from and i also have the microsoft envelope sign at the bottom of my screen as well as the (1) bracket in my Outlook intray, however on certain occassions (i dont know why) when i click in my intray to read the new email it has gone with no trace of where it has gone to - the (1) and the envelope disappear also.
Even with my intray visible the (1) appears but the email does not appear at the top of the page like it should, and as soon as i click anywhere, the envelope sign & (1) disappear again

Recently a friend emailed me saying that she received a email reply stating that i had “deleted my message without reading it”. I hadn’t a clue that she had previously emailed. I dont know why it picks the emails it does to delete as they are from people i know and it is very random. Today, an email came in which Avast identified, i could tell that it wasn’t going to appear in my outlook and whilst trying to figure it out, two other emails appeared which were checked by Avast but did make it through to my outlook intray. Heeeeelp!

If you access your email account through Internet (i.e., webmail), will the emails be there?
Which is your email provider (I mean, @what?)
Did you change the Outlook plugin settings, specially the Heuristic tab of settings?