Try rebooting. I've seen this happen on Vista after an update to SP2 and after resuming from sleep. If that doesn't help, try the firewall repair tool:
http://social.microsoft.com/forums/en-US/onecarefirewall/thread/e67b1989-afd5-47e5-a393-3a551c7c7e60/
And, then there is the hit it with a hammer approach, reinstall OneCare. Make sure you know your subscription live ID before removing OneCare and after installing from http://onecare.live.com activate with the subscription LiveID.
Finally, you can 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 Forum Moderator