Windows Update keeps failing when trying to update .NET 3.5 (error 643)

original title: Windows Update keeps failing when trying to update .NET 3.5

Got the following Installation Failure message starting around January 10 or so.


Windows failed to install the following update with error 0x80070643: Security Update for Microsoft .NET Framework 3.5 SP1 on Windows XP, Windows Server 2003, Windows Vista, Windows Server 2008 x86 (KB2416473).


Went to and tried MS Fix It tool. It said it fixed Update but the Update still failed.


Tried .NET repair solution and removed .NET Framwork 3.5, Now get the following message in th Event Viewer.


Product: Microsoft .NET Framework 3.5 SP1 -- Error 25541.Failed to open XML file C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CONFIG\web_mediumtrust.config, system error: -2147024786.


Windows Update keeps trying to install .NET Framework 3.5 repeatedly.


I have Windows XP with SP3.


What next?


Question Info

Last updated March 12, 2019 Views 7,272 Applies to:

Use this to verify what versions are actually installed -

.NET Framework Setup Verification Tool User's Guide


Normally (well it has in many cases), removing the offending .NET Framework using the removal tool and reinstalling usually fixes the problem.

Unfortunately it appears it's not working in your case.


Verify if any .NET Framework versions are installed, if not then download and try installing the .NET Framework version in safe mode. Can't hurt.


In the meantime, perhaps the MS staff may come up with some further ideas.

No warranties or guarantees.
MVP 1st July 2005- 30th June 2016
WIMVP 1st July 2016- 30th June 2017

Did this solve your problem?

Sorry this didn't help.

Great! Thanks for marking this as the answer.

How satisfied are you with this reply?

Thanks for your feedback, it helps us improve the site.

How satisfied are you with this response?

Thanks for your feedback.