OneCare will update itself to the latest available version no matter when and how you installed it. You can see the current version by opening OneCare and clicking Help and then About...
When installing OneCare on a new PC, you should generally ignore your purchased disc and go to
http://onecare.live.com to install the trial. Once installed, open OneCare, click subscribe and enter the LiveID for your subscription (also shown in help/about as the susbcription ID) to activate and associate the new install with your subscription.
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator