MSN does scan all your email with McAfee before it goes into your mail account, Compuserve does not last I heard.
Yes you still need virus protection, you need to go into Norton's options & select auto protect, then configure your Outlook or Outlook express if thats what your using,within Norton. I take it your opening the email from Compuserve with Outlook at present if so you'll need to set that account up in Norton for it to be protected.
With MSN there is a way to have your hotmail come through your Outlook but I'm not sure how to set that up, If you are able to do it then you can have it protected by Norton also same as your Compuserve email, but not untill it's comong through Outlook.