Problems with installing  .NET Framework 3.5 also can effect all .NET Framework versions

Installation Failure: Windows failed to install the following update with error 0x80070643: 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.

or

EventType visualstudio8setup, P1 microsoft .net framework 2.0-kb958481, P2 1033, P3 1603, P4 msi, P5 f, P6 9.0.31211.0, P7 install, P8 x86, P9 w2k3r2, P10 2721.

Product: Microsoft .NET Framework 2.0 Service Pack 2 - Update 'KB958481' could not be installed. Error code 1603. Additional information is available in the log file C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\Microsoft .NET Framework 2.0-KB958481_20100731_042501630-Msi0.txt.

Product: Microsoft .NET Framework 2.0 Service Pack 2 -- The installer has encountered an unexpected error installing this package. This may indicate a problem with this package. The error code is 2721. The arguments are: CA_ScheduleUpdateAssemblyRB.3643236F_FC70_11D3_A536_0090278A1BB8, , 

This .NET Framework cleanup tool is designed to automatically perform a set of steps to remove selected versions of the .NET Framework from a computer.  It will remove files, directories, registry keys and values and Windows Installer product registration information for the .NET Framework.  The tool is intended primarily to return your system to a known (relatively clean) state in case you are encountering .NET Framework installation, uninstallation, repair or patching errors so that you can try to install again.

There are a couple of very important caveats that you should review before using this tool to remove any version of the .NET Framework from your system:

This tool is designed as a last resort for cases where install, uninstall, repair or patch installation did not succeed for unusual reasons.  It is not a substitute for the standard uninstall procedure

This cleanup tool will delete shared files and registry keys used by other versions of the .NET Framework.  If you run the cleanup tool, you will need to perform a repair/re-install for all other versions of the .NET Framework that are on your computer or they will not work correctly afterwords.