Credit goes to PA Bear MS MVP
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When installing updates using Windows update or Microsoft Update you may
receive Error 0x643 or 0x80070643 (23 Oct-09)
http://support.microsoft.com/kb/976982
=====================
Step #1 => Before doing anything else, write down what .NET Framework
versions are currently installed:
How to determine which versions of the .NET Framework are installed and
whether service packs have been applied
http://support.microsoft.com/kb/318785
Step #2 => Now see Resolution Method 2 in...
When you try to install an update for the .NET Framework 1.0, 1.1, 2.0, 3.0,
or 3.5, you may receive Windows Update error code "0x643" or Windows
Installer error code "1603" (ignore the title, if need be)
http://support.microsoft.com/kb/923100
After running the appropriate/needed tool, (re)install all .NET Framework
versions (but only the ones you wrote down in #1 above) manually using the
download links on the page then run Windows Update manually to install
security updates for same.
NB: If you remove .NET Framework 3.5 SP1 using the tool, reinstalling .NET
Framework 3.5 will also install .Net Framework 2.0 SP2 and 3.0 SP2; then
Windows Update will offer .NET Framework 3.5 SP1.
IMPORTANT! => If you remove .NET Framework 3.5 SP1, reinstall your .NET
Framework versions in the following order:
1. .Net Framework 3.5, then...
2. .Net Framework 1.1, then...
3. .Net Framework 1.1 SP1
NB: KB923100 references this page:
http://blogs.msdn.com/astebner/archive/2006/05/30/611355.aspx. Please read
ALL of it as well as the related page
http://blogs.msdn.com/astebner/pages/8904493.aspx before using the .NET
Framework Cleanup Tool.
Step #3 => Check-in at Windows Update and install any critical security
updates offered.
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