There might be several reasons as to why you can’t install the .NET Framework. I had the problem while installing the framework via the Windows Update.
While trying to install Microsoft .NET Framework 3.5 SP1 Update for Windows 7 x86 (KB982526), Windows Update told me that the update was unsuccessful and gave me the following “useful” error:
You might also get several other errors similar to the following:
- an error of type 0x643
- Microsoft .NET Framework 3.5 (x64) ‘package’:  Error: Installation failed for component Microsoft .NET Framework 3.5 (x64) ‘package’. MSI returned error code 1601
- Microsoft .NET Framework 3.5 (x64) ‘package’:  Error: Installation failed for component Microsoft .NET Framework 3.5 (x64) ‘package’. MSI returned error code 1603
- Microsoft .NET Framework 3.0SP1 (x64) (CBS):  Error: Installation failed for component Microsoft .NET Framework 3.0SP1 (x64) (CBS). MSI returned error code -2146762496
- WapUI:  DepCheck indicates Microsoft .NET Framework 2.0SP1 (CBS) is not installed
It strongly depends on the situation and your system but you might try one of the following solutions:
- You can first try to install the System Update Readiness Tool for Windows 7 which is an August 2011 release (KB947821 – 32-bit (x86)) at this moment of writing. This should solve any inconsistencies in the Windows servicing store which may prevent the successful installation of future updates, service packs, and software. There is also a 64-bit version for Windows 7. Windows Vista: 32-bit (x86), 64-bit (x64).
- If you’re not using Windows Update, then you can try to download and install the framework manually: Microsoft .NET Framework 3.5 Service Pack 1.
- There is also a newer version of the framework which most newer applications use which might also suffice: Microsoft .NET Framework 4 (Web Installer).
- Another thing that you can try on top of all the above is to first remove all the existing .NET framework versions. Do it via Add or remove programs from the Configuration Panel. You can also try the .NET Framework Cleanup Tool.
- Do the following: Start > Run > type “services.msc”. Now check whether the Startup Type of Windows Installer is set to Manual.
- Go to Start > Control Panel > Programs > Turn Windows features on or off. Check now that the Microsoft .NET Framework 3.5.1 is checked on.
- Disable any security/antivirus software while trying to install. Usually this shouldn’t give problems, but it’s worth a try.