Are you seeing a red OneCare circle with a 1 in it? And, when opening OneCare, does it report that the OneCare firewall is off? If so, try this:
Firewall repair tool - http://social.microsoft.com/forums/en-US/onecarefirewall/thread/e67b1989-afd5-47e5-a393-3a551c7c7e60/
If you have a red shield in the system tray with a white x in it, see this thread:
http://social.microsoft.com/Forums/en-US/onecareupdate/thread/297455ff-0484-4dd6-9627-ca1b0e30b015
If the tool does not fix it, or the second link does not apply, then please contact support.
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