Open OneCare. Click on Help/About.
Note the subscription ID shown.
Open your web browser and sign in at http://billing.microsoft.com with that LiveID.
You should see your OneCare subscription and a link to renew.
You may need to update your account information at the Billing site if you originally activated with a retail key.
If that does not help, please contact support
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve