Step 1. It would be super helpful if you could provide the following information at the beginning of your post:

  1. Version of .NET Framework you are trying to install.
  2. Where did you get it (web page, path, application that redistributed the framework)
  3. Is this a pre-release, CTP or beta version? Which? (Beta#, CTP (Month Year), RC, etc)
  4. What .NET Framework Patches do you currently have installed? 

Step 1. Follow Aaron Stebner's blog: http://blogs.msdn.com/astebner/archive/2007/08/01/4172276.aspx

Step 2. If this is a .NET Framework 3.5 install on Vista then you should run the following tool and post the output of the command: http://blogs.msdn.com/aaronru/archive/2007/11/29/net-framework-3-5-installation-validation-tool.aspx

Step 3. If this is a .NET Framework 3.5 isntall on Vista RTM then you should additionally try the following:

Run the following from an elevated command prompt

1.       Winmgmt /verifyrepository

2.       If the repository is reported as INCONSISTENT, run “Winmgmt /resetrepository” (if it fails because of service dependency, run “net stop winmgmt” and try again). If the repository is consistent, skip this step.

3.       Do “mofcomp C:\Windows\Microsoft.NET\Framework\v2.0.50727\aspnet.mof” to compile the failed mof.

If you feel the need to completely remove the .NET Framework and Visual Studio from your machines then you should follow the blog: http://blogs.msdn.com/aaronru/archive/2007/12/10/uninstalling-the-visual-studio-2008-rtm-or-rc.aspx

 Thanks for taking the time to enhance our product.