If you activated the reinstall with the same LiveID, you simply need to renew with your new product key. (The reinstall wasn't needed...)
Go to http://billing.microsoft.com and sign in with your subscription LiveID. (Open OneCare and click help/about to view the subscription LiveID)
Select the OneCare subscription and click on the renew link.
Choose the option to renew with a prepaid PIN or token which is the retail product key.
Enter the 25 character key and proceed.
It can take up to 24 hours for the subscription status to be updated in OneCare.
-steve