If you are getting that error message, it would seem that the key code has already been used. If not by you, unfortunately, by someone else or you ended up with a duplicate retail key.
Is this the first time you are using this key? Had you previously installed and activated a subscription with that key on another PC? If so, you need to activate this PC with the same LiveID used originally, not the key. Since you are following the activation process, this means the copy on your PC is in trial mode and that the trial period is about to expire, not your subscription.
You can contact support to find out the LiveID for your subscription (associated with that key) or to get help regarding the fact that you may have a duplicate key.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve
