Open OneCare.
On onecare circle verify which among your computers have the problem.
You should work directly on the computer which have the problem.
Onecare circle is only a monitor, it reports if any computer on the circle needs some attention.
To turn on Virus and Spyware monitoring
Verify if the date and time setting of your computer is correct.
Save and run this tool. Windows Live OneCare Anti-Malware Repair tool
AntiMalwareRepair.exe
http://www.microsoft.com/downloads/details.aspx?FamilyID=70adca25-950c-4b1b-a5e2-297e80aa67b1&DisplayLang=en
If this won't fix the problem, contact support.
How to reach support (FAQ) -
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2