If you have just installed and activated OneCare and it is reporting that your subscription is expiring, check your PC date and time. Also, verify your subscription status by signing in at http://billing.microsoft.com.
Your subscription is for 12 months from date of activation.
If you need help, please contact support -
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