Thanks for the information so far. When I put Avast on my older computer it kept giving me a message that port 110 was already in use (which it was, 1st Email Antivirus listens on port 110). On my main computer I also have 1st EAV + ChoiceMail which uses port 110 on localhost [it also uses localhost for outgoing messages as it automatically adds people I write to to my white list] and passes messages via port 109 to my e-mail client [if I understand it correctly]). I presume I would get a similar error message if I put Avast on my main machine. I suppose to make life simpler, I could turn off e-mail scanning because 1) 1st EAV removes all scripts, web bugs, etc and zips up all attachments (apart from any exceptions I specify), 2) ChoiceMail can be set to block all message with certain attachments types (so I could set it to block all messages with zip attachments passed to it from 1st EAV) and 3) my e-mail client Courier can also be set to remove scripts, etc, so I have three layers of security even without any virus scanner.
One other thing, does anyone know which file I need to use for download managers (I seem to remember that when I selected browse, I saw that there were several exe files in the Avast folder, but I didn't know which one to select)?