Sign in with your LiveID for your subscription (Help/About in OneCare if it is still installed) at http://billing.microsoft.com
You should be able to renew from there.
If you continue to get that error, and can't renew, please contact support to have them look into the problem -
How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
If it fails to validate your subscription, select the option that you are using a trial or beta copy and you can proceed to email support without validation once you've signed in.
-steve