On my developer pc I had a .NET Framework 4.0 RC installation, over which I installed the final release, without removing the RC version. When I wanted to set things right and uninstall all of .NET Framework 4.0 the normal way through control panel, it failed to do so. Also I could not repair or reinstall it.
Via google I found Aaron Stebner's blog where one can download a handy utility '.NET Framework Cleanup Tool'. This tool helped met in removing the last remains of version 4.0. After the cleanup I was able to install the .NET Framework 4.0 again and continue my real work.
The tool can be found here