We have released a tool that troubleshoots common issues with the setup of or with updates to Microsoft .NET Framework setup installation. Additionally, this tool tries to resolve these issues by applying known fixes or by repairing the installed product.
More details can be found here :
Download: .NET Framework Repair Tool – Microsoft Download Center – Download Details
Note: The current version of the tool supports repairing the .NET Framework 4, this will be extended to include other versions of the .NET Framework in the future.