If you reinstalled OneCare and then activated with the same LiveID as an existing subscription, it inherits the subscription status. You didn't need to reinstall, but simply renew with the retail key.
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