WindowsUpdate of .Net 3.5 SP1 (KB951847) failed to install several times. .Net 3.5 SP1 already installed

I have a Windows 2003 server x86, which I have manually installed .Net 3.5 SP1, but now, the Windows Update:

Microsoft .NET Framework 3.5 Service Pack 1 and .NET Framework 3.5 Family Update for .NET versions 2.0 through 3.5 (KB951847) x86

failed to install with error:

Error Code: 0x80070643
Try to install the update again, or request help from one of the following resources.

What to do?

Have you tried running the installation package outside of Windows Update? You find the package in %WINDIR%\SoftwareDistribution\Download\Install once you've tried to install it. Normally, this way you get better error messages

Updated: You may try this: Stop service Windows Updates from services.msc and delete or rename SoftwareDistribution folder afterwards. Start Windows Updates again and it should recreate the folder structure (at least when new updates arrive)