2. If a repair does not help, then go to the Add/Remove Programs control panel and attempt to uninstall the version of the .NET Framework that is causing problems on the system
3. If you are attempting to uninstall the .NET Framework 2.0 and it fails, download and run the Microsoft .NET Framework 2.0 Registration Correction Tool (also named clwireg.zip and described in more detail in this blog post), and then try to uninstall the .NET Framework 2.0 again. If you are encountering errors uninstalling a version of the .NET Framework other than 2.0, you can skip this step because this tool only works for the .NET Framework 2.0.
4. If uninstall still fails from Add/Remove Programs, download the .NET Framework cleanup tool and choose to remove the version of the .NET Framework that is causing problems on your system
Download and install the version of the .NET Framework that you cleaned up in step 2 or 3. Here are some download links for various versions of the .NET Framework:
.NET Framework 1.0
.NET Framework 1.1
.NET Framework 2.0
.NET Framework 2.0 with SP1
.NET Framework 2.0 with SP2
.NET Framework 3.0
.NET Framework 3.0 with SP1
.NET Framework 3.5
.NET Framework 3.5 with SP1
.NET Framework 4
Source: what to do if other .NET Framework setup troubleshooting steps do not help :By Aaron
