As the free trial period nears expiration, your OneCare status will change to At Risk. This also happens when a paid subscription is near expiration and a subscriber has not yet renewed. Your system is still protected when in this status, as long as this is the only reason for the at risk status. Opening OneCare and viewing the actions to take at the top of the page will confirm the reason for the status. Once you subscribe, the free trial period ends and your subscription begins - and your status will change to green/good, usually immediately, sometimes within 24 hours, depending on how you subscribe.
-steveMicrosoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator