Had a long evening trying to install the ASP.NET MVC 3 Tools Update. Web PI installer stated that everything was installed successfully, even though the Visual Studio and the Add/Remove Programs stated otherwise. Downloading and running the MSI-package gave more hints about what was wrong because the installer would start the rollback as soon as it tried to install ASP.NET Web Pages. The installer’s log mentioned that the installation failed because of an error 0x80070643.
Thankfully I wasn’t the only one with this particular problem and Googling brought up a good blog about the problem: http://blog.williamhilsum.com/2011/03/error-0x80070643-installing-aspnet-mvc.html
Following the advice on the last update of the post to modify the Windows’s Registry helped to solve all the problems.