Did you purchase from within an installed trial copy of OneCare? Or did you purchase via the web site? If it is the former, entering your LiveID and payment information starts your subscription and OneCare automatically activates by receiving credentials behind the scenes as part of the process. If you go to Help/About in OneCare it will show your subscription status. You can also confirm your status at
https://billing.microsoft.com If you purchased via the web, you need to open OneCare (or install the trial from
http://onecare.live.com ) and then subscribe/activate by signing in with the LiveID you used to purchase.
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator