Unable to install Security Update for Microsoft .NET Framework 3.5 (KB2736416) / Error code code 643

OS Name: Microsoft Windows Vista Home Premium

System type: X86-based PC

Version: 6.0.6002 Service Pack 2 Build 6002

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I am unable to install "Security Update for Microsoft .NET Framework 3.5 SP1 on Windows XP, Server 2003, Vista, Server 2008 x86 (KB2736416)."

I get the message: "error(s) found / code 643 / windows update encountered an unknown problem."

This is what I have tried to fix the problem, per Microsoft:

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• Run the Microsoft Fixit from the link.
  
• http://support.microsoft.com/mats/windows_update/
  

When I click the download button I get something called "windowsupdate.diagcab." I don't know what that is or how to open it.

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• Fix MSI software update registration corruption issues
  

I download "MicrosoftFixit.wu.RNP.135281810351121704.4.2.Run.exe" but when I try to run it I get the message "Troubleshooting cannot continue because an error has occurred. We're sorry, but an error occurred while downloading one or more components and

the troubleshooter cannot be run."

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• Repair the .NET Framework
  
• Select Repair .NET Framework 4 Client Profile to its original state, and then click Next.
  

I did this but it did not seem to make a difference.

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• Downloaded and installed System Update Readiness Tool for Windows Vista (KB947821)
  

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I don't know what else to do. I tried Windows Live Chat only to find out it would cost me $99 to get help with this problem from Microsoft. Surely Microsoft can do better than that! Any help would be appreciated! Thank you.

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