Try reinstalling the Windows Defender program; if the problem is not solved, check the program directory for mpclient.dll by going to Search, All Files and Folders, More Advanced Options and selecting Search Hidden Files and Folders. If the file is not available in the program directory, you will need to download it either directly from Microsoft or from a reliable third party web source.
edit: to manually replace it if no go on the reinstall... dl it first from a safe place or copy it off another machine, then
- Save MpClient.dll to C:WindowsSystem32winsxsamd64_security-malware-windows-defender_31bf3856ad364e35_6.1.7600.16385_none_b3b1a27171e01f6c
- Type regsvr32 MpClient.dll and press Enter in CMD window
Edited by TsVk!, 26 November 2013 - 05:36 PM.