Windows Live OneCare is not compatible with Windows 7.The messages listed in the "Symptoms" section can occur if Windows Live OneCare is running during the upgrade to Windows 7, or they can occur after you restart the computer.  Microsoft is advising users to completely uninstall Windows Live OneCare prior to upgrading to Windows 7. No word yet from the company whether it plans to introduce Windows 7 support, but such a scenario is highly unlikely, seeing as OneCare is a case of a “dead antivirus walking.”

More info: KB968420