I believe McAfee Virus Scan Online checks for update when the user goes online without any intervention from the him/her. Mine does, anyway. I think I paid USD 29.95 per annum for the service or something like that.
Another thing I found useful was to select "Do not allow attachments to be saved or opened that could potentially be a virus" in Outlook Express. I don't know how much safer this will make things, but at least it gives me some added security. To those knowledgeable on the matter, how much safer does ticking this box make?