Check for new definitions" warning after you install Windows Defender
After you install Microsoft Windows Defender, you may receive the following "Check for new definitions" warning:
Definitions allow Windows Defender to detect the latest harmful or unwanted software and prevent from running on your computer.
In addition, the Check for updates now button is unavailable.
If you check for updated definitions by clicking Check for Updates under the Windows Defender Help option, Windows Defender reports that no updates are available.
This problem occurs because Windows Defender could not automatically update the definitions.
Solution at http://support.microsoft.com/?kbid=915105 via Calendar of Updates