What's the current state of OneCare on the PC? Do you have a link to subscribe? If not, click Help/About OneCare.
Does it show your active subscription ID or does it show that you are in trial mode?
If you have a link to subscribe, click it and sign in with the LiveID that you started your subscription with and it should activate the locally installed copy.
If it shows your Subscription ID in Help/About and OneCare is in green status, there's nothing else for you to do.
-steve