Issue

KB2468871 fails to install on Windows 7 64bit with error code 643.
Full name of the update: Update for Microsoft .NET Framework 4 on Windows XP, Windows Server 2003. Windows Vista, Windows 7, Windows Server 2008. Windows Server 2008 R2 for x64-based Systems (KB2468871).

Some updates were not installed
Error(s) found: Code 643 (Windows Update encountered an unknown error)

Resolution 

The issue was caused by a missing System.Xml.Serialization.dll file. To resolve:

• Download KB2468871 manual installation file (NDP40-KB2468871-v2-x64.exe) from Microsoft.
• Open the file with 7-ZIP and navigate to NDP40-KB2468871-v2-x64.exe\NDP40-KB2468871.msp\QFEGDR.cab\
• Extract file called System.Xml.Serialization.dll.amd64 and rename it to System.Xml.Serialization.dll
• Copy above file to following locations on the affected machine:
  • C:\Windows\Microsoft.NET\Framework\v4.0.30319
  • C:\Windows\Microsoft.NET\Framework64\v4.0.30319
• Try to install KB2468871 again.

The updates were successfully installed

September 2016
Windows 7 Pro 64bit
.NET Framework 4.0