You really only need AV software enabled for checking/sending e-mails where the real risk of being landed with a virus lies.
I have Norton AV2002 and it is configured in this fashion.
But I do update antivirus definiations at least once a day, which has paid off several times in the last 10 days or so when it has stopped these nasties in their tracks.
I also have the Outlook Express Preview Pane disabled at all times - one of the key causes of setting a virus loose. Even clicking on an e-mail to delete it can set the virus in action if the Preview Pane is active.