If you activated OneCare with a retail key, but it still shows you have not activated, check at http://billing.microsoft.com to see if your subscription is active. If so, reinstall OneCare and activate with your LiveID. If you did not activate, but you are getting an error that the key has been used, your need to either go back to the retail store for a replacement, or contact OneCare support - use the option for a trial/beta user to open an email request.
How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve