Please first try the workaround in this article: Assembly load failure installing the 2.0 Framework which described an issue similar to yours.
If this does not solve your problem, you can try using the following steps to solve this problem:
- Download the .NET Framework cleanup tool and choose to clean up 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 1 (such as the .NET Framework 1.0, .NET Framework 1.1 or .NET Framework 2.0)
- (optional) Download and run the .NET Framework verification tool to double-check that all .NET Framework files are correctly installed
- Download and install the desired .NET Framework service pack (such as .NET Framework 1.0 SP3 or .NET Framework 1.1 SP1) by running the setup package directly instead of using Windows Update. Running it directly will allow the service pack setup to display error dialogs instead of having Windows Update suppress them
For more details, please visit What to do if other .NET Framework setup troubleshooting steps do not help.