If your subscription has not expired, 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 token which is the retail product key.
Enter the 25 character key and proceed.
Once you have completed this, OneCare will update the subscription status the next time it communicates with the server.
If your subscription has expired, uninstall the current copy and install the new copy from http://onecare.live.com and activate with your LiveID and retail product key.
If you encounter any problems, contact support -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve
