Susan and UTCSTOM, please check your computer date and time. If it is a future date, that would explain the expiration.
You can also verify the status of your subscription by signing in with your LiveID at http://billing.microsoft.com - the LiveID for your subscription is under Help/About in OneCare, listed as Subscription ID.
If your subscription is current on the Billing site, and the PC date and time are correct, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
Note also, that there was a problem with subscriptions that were renewed using a retail key, if the original subscription was created with a retail key. That was resolved as of 12/21/07 and appeared on 12/18. If your subscription fits this description, then please open OneCare and click Check for Updates to force a subscription check along with a signature check.
-steve