You did not need to uninstall/reinstall. You simply needed to renew with the retail key.
Now that you reinstalled, by activating with the same LiveID, the new install inherited the subscription status of the existing subscription and you were not prompted for payment or a retail PIN/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.
Once complete, your subscription will expire in April 2009.
-steve
