If you are trying to activate OneCare with the LiveID that you know is correct and can validate this by using the Live ID to sign in at
https://billing.microsoft.com to see your current OneCare subscription, then you may have changed your password to be longer than 8 characters after you initially activated OneCare. There is a defect in the process that causes a login for activation with passwords over 8 characters to fail. Change your password at
http://account.live.com and activate from within OneCare. Feel free to change the password again after activation is complete.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator