Is OneCare reporting this or do you have a red shield with an X inside within the System Tray? If OneCare is reporting it and the OneCare icon is red when you start the PC, it is apparently having trouble getting started, possibly due to system resources.
If it is the red shield, then the problem is actually with the Windows Security Center. If you do nothing at all, including opening OneCare, and the message clears and the shield disappears from the system tray, it is also due to a resource issue at startup and OneCare is not reporting status to the Security Center before it throws the warning.
-steve