Welcome to the forums, even though your post is in the wrong one, hopefully one of the moderators will move it to the avast 4.x Home/Pro forum,
http://forum.avast.com/index.php?board=2.0.
If you have no intention of using the email function of Outlook 2003 you can terminate the avast Outlook/Exchange provider.
Not only do I not recommend you disable the Internet Mail provider, I would suggest you set to High sensitivity. If you have an undetected or hidden trojan spambot on your system (they come with their own SMTP process) the Internet Mail provider would notice multiple emails in a given period. This could be your first and only indication your infected and sending out spam.
I would tend to leave them all ticked as it doesn't use any additional resources (nor will it run faster) and if you happen to try one later you don't have to remember to enable it inclusion for monitoring.