It is also possible that your password is longer than 8 characters, while it was shorter when you originally subscribed with that LiveID. If so, change it at http://account.live.com.
If the message is that your ID or password are incorrect, then you are likely using the wrong LiveID for your OneCare subscription.
In either case, if you are still unable to proceed, please contact support per the link Jim provided.
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator