First, try going to Windows Update manually from the Start Menu in Windows and then installing all offered critical/important updates. If that doesn't clear the warning from OneCare, the contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
If you have problems with one or more of the updates offered failing, you can contact OneCare support or start here:
Windows Update Solution Center - http://support.microsoft.com/ph/6527
I'm moving this to the Tune-up forum as it isn't an update to OneCare that you have an issue with, but Windows Update and/or the interaction of OneCare with that.
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator