Your LiveID for your subscription is the one you need to log in with. If you changed your subscription LiveID, then this PC still knows your old one and you may need to reinstall OneCare on it.
Check Help/About... in OneCare on each of your computers to see the subscription ID. Also, check at
https://billing.microsoft.com for your susbcription. Note that changing your email address does not change your LiveID for your subscription. If you changed your LiveID at
http://account.live.com you may have created an issue with your paid subscription and you may need to contact support.
Typically, you cannot close a LiveiD or migrate to a new LiveiD when you have paid services assocaited with it.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator