Go to Start/run.
Copy and paste this whole line: ( with ")
"C:\Program_Files\Windows Defender\MpCmdRun.exe" -RemoveDefinitions -All
then:
"C:\Program_Files\Windows Defender\MpCmdRun.exe" -SignatureUpdate
This step may not work in many cases. In many instances the issue is with
the 1.0.0.0 definition set, so rolling back to that version may not solve
the problem. A better strategy may be to follow KB 915105
(http://support.microsoft.com/?kbid=915105) to uninstall the signature MSI
completely.
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