First, check your PC date and time.
Next, open OneCare and click on Check for Updates.
If your account still shows expired after the update check, sign in at http://billing.microsoft.com and verify that the account is active.
If OneCare on the PC still fails to reflect the correct account status, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve