My PC has not been able to install Security Update for Microsoft .NET Framework, Version 4.5.1 (K2898869). The error code is 643. "Window Update encountered
an unknown error." My PC was restored on 7/25/14 because of a virus attack. I just reinstalled Malwarbyte successfully. My PC works fine now. Microsoft Security Essentials works too. My system: Window 7 64 bits.
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