.NET Framework 

Windows 8

Installing the .NET Framework 3.5 on Windows 8
http://msdn.microsoft.com/en-us/library/hh506443.aspx

ASP.NET 2.0 and ASP.NET 3.5 do not work after you uninstall ASP.NET 4.5 in Windows 8 or Windows Server 2012:
http://support.microsoft.com/kb/2748719

Windows 8, Windows Server 2012 blocks Framework 4.0 Downloaded Install
http://support.microsoft.com/kb/2765375
“Microsoft .NET Framework 4 is already part of this operating system ……. “

Error codes when you try to install the .NET Framework 3.5 in Windows 8 or in Windows Server 2012
http://support.microsoft.com/kb/2734782
0x800F0906 - CBS_E_DOWNLOAD_FAILURE
0x800F081F - CBS_E_SOURCE_MISSING
0x800F0907 - CBS_E_GROUPPOLICY_DISALLOWED

Windows Vista, Windows 7 & Windows XP

Error Code: 0x80131700
Error Code: 0x643
Error Code: 0x80070002
Error Code: 0x66A

You will need to reinstall Framework –

Error code "0x8007066A" when you use the Windows Update or Microsoft Update Web site to install .Net Framework updates:
http://support.microsoft.com/kb/2507641/

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"
http://support.microsoft.com/kb/923100

When installing updates using Windows update or Microsoft Update you may receive Error 0x643 or 0x80070643
http://support.microsoft.com/kb/976982

Vista
http://windows.microsoft.com/en-US/windows-vista/Windows-Update-error-80070643 

Windows 7
http://windows.microsoft.com/en-US/windows7/Windows-Update-error-80070643

Use the above as guides to fixing the problem.

You can use this to verify what versions are actually installed on your computer-

.NET Framework Setup Verification Tool User's Guide
http://blogs.msdn.com/b/astebner/archive/2008/10/13/8999004.aspx

.NET Framework Cleanup Tool:
http://blogs.msdn.com/b/astebner/archive/2008/08/28/8904493.aspx

Reinstallation order (if required):

1. .NET Framework 3.5 SP1 (this will install the .NET Framework 2.0 SP2 and 3.0 SP2 )
2. .NET Framework 1.1 (if you have any programs installed that need it, this blog post can help you decide whether or not you need it)
3. .NET Framework 1.0 (if you have any programs installed that need it, this blog post can help you decide whether or not you need it)
4. .NET Framework 4

For further information please see: http://blogs.msdn.com/b/astebner/archive/2010/12/22/10108400.aspx

You can try if appropriate just repairing only the affected .NET Framework versions - http://support.microsoft.com/kb/976982

In many cases repairing .NET Framework 4 is sufficient to fix the corruption but in some cases, uninstalling and reinstalling .NET Framework 4 is required
(Ref: http://support.microsoft.com/kb/976982)

1. Close any currently running applications.
2. Click Start, and type Uninstall in the Search box, and then click Uninstall a program.
3. Double-click Microsoft .NET Framework 4 Client Profile.
4. Select Repair .NET Framework 4 Client Profile to its original state and click Next.
5. Once the repair is complete, click Finish.
6. Click Restart Now to restart your computer.
7. Install the latest Windows Updates by clicking the Start button, then type Windows Update in the Search box, then click Windows Update.

1. Close any currently running applications.
2. Click Start, click Control Panel and then click Add or Remove Programs.
3. Click Microsoft .NET Framework 4 Client Profile from the list of installed programs, then click Change/Remove.
4. Select Repair .NET Framework 4 Client Profile to its original state and click Next.
5. Once the repair is complete, click Finish.
6. Click Restart Now.
7. After you log back into Windows, click the Start button, then type Windows Update in the Search box, and then click Windows Update.
8. Click Check for updates. After the scan is complete, click Install Windows Updates.
9. Follow the onscreen instructions to install the Windows Updates.

Troubleshooting .NET framework install failures
http://blogs.msdn.com/b/varungupta/archive/2010/06/22/troubleshooting-net-framework-4-install-failures.aspx

Search downloads for .NET Framework if required:
http://www.microsoft.com/en-us/download/search.aspx?q=.net+framework

Note: Mailbag: What version of the .NET Framework is included in what version of the OS? http://blogs.msdn.com/b/astebner/archive/2007/03/14/mailbag-what-version-of-the-net-framework-is-included-in-what-version-of-the-os.aspx

How to determine whether a computer is running a 32-bit version or 64-bit version of the Windows operating system
http://support.microsoft.com/kb/827218

How to repair the .NET Framework 2.0 and 3.0 on Windows Vista (applies to Windows 7 also)
http://blogs.msdn.com/b/astebner/archive/2007/03/26/how-to-repair-the-net-framework-2-0-and-3-0-on-windows-vista.aspx

Unified .NET Framework Troubleshooting Guide
http://blogs.msdn.com/b/astebner/archive/2005/08/22/unified-net-framework-troubleshooting-guide.aspx

Other information -

1. Benefits of the Microsoft .NET Framework
   http://support.microsoft.com/kb/829019

2. How to determine which versions of the .NET Framework are installed and whether service packs have been applied
   http://support.microsoft.com/kb/318785
   This article describes how to determine which versions of the Microsoft .NET Framework are installed on your computer.

3. How to repair an existing installation of the .NET Framework
   http://support.microsoft.com/kb/306160
   You may need to repair your installation of the .NET Framework if you upgrade your operating system or if the current installation of the .NET Framework becomes corrupted. This article describes how to do so.

4. How to troubleshoot Microsoft .NET Framework 1.1 installation issues
   http://support.microsoft.com/kb/824643

5. .NET Framework 1.1 not supported on Windows 7 nor Windows Server 2008 R2
   http://support.microsoft.com/kb/2489698

6. Error message when you try to install the .NET Framework 1.1 Service Pack 1 Update from the Windows Update site on a Windows XP-based computer: "Updates were unable to be successfully installed"
   http://support.microsoft.com/kb/922377

7. You may receive an "Error 1935" error message when you use the Netfx.msi Windows Installer package to install the .NET Framework 1.1
   http://www.support.microsoft.com/kb/839547

8. How to troubleshoot Microsoft .NET Framework 2.0 installation issues
   http://support.microsoft.com/kb/908077/

9. The .NET Framework 2.0 SP1 installation fails on a computer that has the .NET Framework 2.0 installed and that is running Windows XP, Windows Server 2003, or Windows 2000:
   http://support.microsoft.com/kb/951950

10. Microsoft .NET Framework 2.0 Registration Correction Tool on Windows XP, Windows Server 2003, and Windows 2000:
    http://www.microsoft.com/downloads/details.aspx?FamilyID=0ba6038c-061e-4b4a-9be9-96a323701260&DisplayLang=en
    On a computer that has Microsoft .NET Framework 2.0 installed, you may receive a an error while installing Microsoft .NET Framework 2.0 Service Pack 1 and the installation may fail.

11. Do I need still need older versions of the .NET Framework on my system after installing .NET Framework 3.5 SP1
    http://blogs.msdn.com/astebner/archive/2009/04/20/9557946.aspx

12. Products or updates may not be installed correctly when Microsoft .NET Framework 4 or updates for Microsoft .NET Framework 4 are installed after the other product or update installs and a restart is pending
    http://support.microsoft.com/kb/2473228

13. An update for the Microsoft .NET Framework may fail to install when the Microsoft .NET Framework 4 is installed and a restart is pending
    http://support.microsoft.com/kb/2431208

Last updated 18th August 2012