Don't reinstall OneCare and do the following any time before expiration (as long as your subscription has less that 12 months remaining), but make sure to do it at least a few days before.
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 if you are renewing within 30 days of expiration and 5 days if renewal was performed between 90 and 30 days before expiration.
If you encounter any problems, contact support -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve
