If you created a new LiveID to activate OneCare and are now using this new LiveID in Windows Live Messenger, too, then you'll need to add your contacts again as the contacts were associated with your old LiveID. Adding them into your new account may still require your contacts to approve you, depending on their contact settings. You can also sign in to Messenger with your old LiveID no matter what you used with OneCare.
Support for Windows Live Messenger, Hotmail and other Windows Live programs and services: http://support.live.com
Help topics at http://help.live.com
-steve