I read your post but in the help file it says that it does receive the updates automatically through Windows Update which is set to install updates automatically. Maybe that has changed since MS made the definition updates optional in WU instead of important for their own obscure reasons. I do know that if the defs for Defender are too outdated, Security Center will let you know and give an option to update through there.
I have turned Defender back on and turned on the real time protection and set it to do a quick scan on a weekly basis. I also set it to check for new defs before scanning. We'll see what happens. If there are problems, I can always turn it back off. The user of the machine goes to online gaming sites like Yahoo and Gamehouse, uses Yahoo and MSN messenger, and also frequents Facebook much more frequently than I do so maybe she needs the extra protection (if there really is any).
One more question, she uses Yahoo web mail for all of her email. Does the mail shield in Avast! scan that or do the other shields like web, network, and file take over for web mail sites?